Trucker, the others make a fine point and i understand how you feel because sometimes the exact same thing happens to me.
I will get all worked up and excited to come home and sit down and pick a plane, departure point, destination and make it happen and by the time i do all that the excitement fades and i almost ask myself "Whats the point of all this really?" my logic is that in the end i will have sat stationary in my computer room and pretended to fly an airplane from point a to point b for no reason other than because
when used to get released from a deployment to work and i came home after 9 months of not having sat in front of my own computer the first thing bet your arse I did was started zooming around the virtual skies in FSX, i kept the sky hot completing at least 5 flights a week for about a month... then one day i unplug the yoke and remove it from the desk and it sits idle for a couple months
the question is - why?
i think the answer is a lot simpler than we think - we burn out on flying the same flights from the same airports (or similar ones) for ultimately no point really. after realizing that there was no point, and little challenge to flying around in excellent weather all the time with no passengers or cargo and no immersion into what i was doing... so i developed my own strategies for making FSX more enjoyable by adding challenges and items to make FSX more immersive and for the most part, they do bring me out of the funk (bear with me because some of these have been mentioned)
1 - Air Hauler: The program "Air Hauler" is a stand alone program that runs outside of FSX. it allows you to create a Cargo Company, Pick a hub, hire pilots of varying skill levels and salary demands, you can customize the names of your pilots, you pick the type of aircraft you want to fly from your virtual fleet, customize their tail numbers, you can upload a company logo... the program tracks all of your company expenses like fuel costs, insurance costs, labor costs, aircraft payments or lease payments, the rent payments for your hub etc one of the neatest features in Air Hauler is that it provides a list of "Jobs" suitable to the aircraft type you have chosen and each job has a compensation total. One job might be to fly 12,000 lbs of Computer parts from your hub to Kansas City, Missouri, another job might be to fly 8,000 lbs of tools from Little Rock, Arkansas to your hub... you can customize the list to provide a lot of jobs or only a hand full at a time. And there is some strategy to route planning because your aircraft use fuel and fly these routes in real time so you wouldnt want to Fly someone from your hub to Kansas City to drop off computer parts then dead head to Albuquerque to pick up the car parts to bring them back to your hub. But perhaps the coolest thing about Air Hauler, lets say you assign your pilots 10 flights and you pick up a flight from your Hub to Atlanta carrying 7500 lbs of animal feed. When you fuel up the plane, load the cargo and hit fly... Air Hauler launches FSX with the appropriate aircraft, fuel load and weight and balance AND it inserts your AI aircraft complete with the custom tail numbers and they will be flying the routes within FSX you assigned them. when you reach your destination, simply accept the pop up to unload cargo and air hauler comes to the forefront where you can load the next cargo for the next run, get paid for the job you just completed, pay for your fuel and refuel the aircraft. it gives FSX "purpose" - Air hauler two is expected to release soon
2. Saitek Yoke, Rudder and Throttle quadrant: One of the best things i did for my FS Experience was buy this thing. Since i find myself flying very few HOTAS fighter aircraft i thought what is the purpose of my flying this 737 around with a basic joystick and fighter pilot throttle? The yoke and throttle quadrant added so much more of a feel of flying a real airplane that i cant do without it now. plus, it has several buttons for autopilot disconnect, starter 1 and 2, fuel flow 1 and 2, gear up, gear down, flaps up and down etc etc all rocker switches on the throttle quadrant, all levers etc are customizable - it rocks
3. Track IR: as previously mentioned by Oldairmal - the track IR is as close to on screen virtual reality as you can get until we all spring for the "oculus rift" simply put on a baseball cap with a small reflective spot on the bill, and clip the small IR sensor to the top of your monitor and you are done. Its like having perpetual mouse look enabled only instead of moving the mouse to look down at the radio stack or look over out of the window... you just turn your head. It actually is very natural feeling and works quite well.
4. FSReal WX Lite: this is a great program for loading real world weather into FSX... like an idiot, i always wondered why my FSX real world weather was never quite like what was actually being seen out my window of my home... one day i landed in the middle of a violent thunderstorm and noticed that the real world weather report for that airport was clear and clean... thats when i googled it and realized DUH... real world weather support ended a LONG time ago. FSReal WX Lite will read out every single weather reporting station within something like 50 miles of your aircraft and adjust each accordingly in real time in game... the result is some fairly realistic weather that adds a bit of challenge to each flight.
5. FS Dream Team Ground Services: i love this little addition because whether you fly a cessna 150 or a 737 it applies and modifies itself to meet the needs of the aircraft. a marshaller with wands will marshal you to parking spot, baggage loaders with animated ground crew will load bags onto the conveyor belt, caterers will drive up and load your plane up with snacks and cola, the flight attendant will greet passengers as they board... its very immersive especially when you routinely fly airliners. while mostly eye candy... it lends more realism to airline operations because you dont just jump in and start up, push back and takeoff. You have to order fuel, catering, request baggage loading to start and passenger boarding to start etc so you could literally be waiting there for 5 minutes running through your pre start activities while all this happens
6. Get yourself a first officer: No - not one of these voice activated programs... a real one. even though i have not done this in a very long time... FSX has left and right seat capability, you can also hand over the controls on multi crew flights. So setup a flight from Atlanta, Georgia to Las Vegas Nevada in your Delta Airlines 737 and fly one leg out and one leg in... the FO will fly it there and you handle radios or whatever and then switch where you fly the next leg. this is great when you have a mic on both ends because you can engage in idle chit chat, or run check lists back and forth, make speed calls etc with a real person
7. hand fly the aircraft. Its super easy in real life and its super easy in the sim to fire up "george" and let the automation do the flying while you gawk out the window at the neat new scenery pack you just downloaded. Nothing wrong with this, but set yourself a rule... i normally do airline flights... so my rule of thumb i follow is that unless nasty weather or some other factor makes the flight particularly a challenge - I'm hand flying the aircraft trough 10,000 ft in the climb before George clocks in. generally i will take over well before my final vector to an approach and hand fly the final vectors and join the localizer and glideslope by hand all the way to the numbers. this is much more fun when you combine this with the yoke and throttle and rudder pedals.
8. Master a new aircraft... every FSX player out there has a go to aircraft they operate more than any other in the hangar. for me, the American Airlines new paint schemed 737-800 is my bread and butter, next to her is the wings of power B-17G Flying Fortress followed by Carenado Cessna 152... red with wheel pants. The point is, when i transition to the B-17 with her super realistic systems and interactive crew... there are a lot of things that dont apply to her like they would the 737. Not only do i have to rearrange my throttle quadrant to that of a 4 engine heavy bomber, but i had to learn how to start the damned thing

what the nuances of her engines were, how it handled etc. This is especially true of A2A brand payware aircraft because their engines wear out in game and you have to overhaul them, change their oil and filters, change their tires, fuel them up because A2A's aircraft will "save state" within FSX how many engine hours, how much fuel, switch positions etc so when you hop in your B-17 or your Cessna 172 in a few days... it will have the exact same amount of fuel you left it with. some aircraft from A2A you actually have to go through their preflight inspection as parts will wear out and fail in flight. Some of the A2A aircraft allow you passengers, so when my pretend american airlines 737 captain bids two weeks vacation, he goes down to the flight museum in Galveston, Texas and sends the B-17 on a three or four day airshow tour throughout the southeastern USA... or he loads a blond into his Cessna 172 and flys her to a weekend in the Bahamas
9. hand plan and navigate: Sometimes when i feel like going low and slow i will fire up skyvector.com on my laptop and set it next to me on the computer desk... i will plan a vfr flight down low and slow. No GPS wizardry allowed. i have to find that little town i chose as a checkpoint, or that funny shaped lake or crooked S turn in a highway - most of the things on a sectional chart like the online sectionals at skyvector are actually in the sim so when you fly over the little black box on the chart that says "Factory" if you look down in FSX there is very likely to be some sort of representation of a factory there. Its pretty rewarding to hand fly the little Carenado Cessna 152 from northern California to Long Beach using flight following and a good old fashioned chart because not only will it put the flying skills to the test and make sure you are up to speed on spotting things visually, maintaining heading, correcting for crosswind and VOR navigating... but it will also sharpen those skills.
10. Turn it off for a while. this is the lowest cost and one of the more effective ways to keep FSX fresh. just turn it off. for just 2 or three days play something else and completely ignore her. Absence makes the heart grow fonder they say