Heya ciuwilliam,
How I do the pushback is to press and hold down the Shift key and then the P key. I hold down both keys simultaneously for about 2 seconds then release both keys. Works like a charm every time for me. If you are knowledgeable in installing Add-Ons, you may wish to consider placing an additional panel like I have for pushback using the Groundhandling Gauges for FS2002 / FS2004 aircraft By Rob Barendregt available here (you may need to create a username/password to login):
http://www.flightsim.com/cgi/kdl/main/51/rcbgh-32.zip?lfPv600CThis Add-On allows you to control pushback and taxiing with the click of a mouse. Very useful.
When I wish to start a new flight in FS 2004 I will press the Alt key and click on Flight in the Main FS 2004 Menu. I then can click End Flight. When the small window appears I can then click on the End Flight button. This will take me to the FS 2004 Setup Screen.
Essentially this will reset the FS 2004 program back to scratch.