Note: This will work about 95% of the time on S60 3rd edition phone models. I have not tested it on other models but feel free. This will not work on phones with a landscape screen (E series). This will work with any MIDlet by the way, not just games.
1. Using an archiver such as WinRAR, open the jar file.
2. Once you have opened the jar, you will see a folder called 'META-INF'. Open this folder.
3. In the META-INF folder there will be one file called 'MANIFEST.MF'. Drag this file onto your desktop and open it in notepad.
4. Now you must know 2 things to edit the file. First, you must know the original resolution of the game. Ususally this information can be found on the author's website. Second, you must know the resolution of your screen. Then type the following two lines at the end of the text in the MANIFEST.MF file:
In the example above 'www,xxx' represents the original resolution of the game. 'yyy,zzz' represents the resolution of your phone's display.
In the picture example below, the original game resolution is 176x208 and the target resolution of my screen (N80) is 352x416.
5. Now save the changes in the MANIFEST.MF file and drag it back into the META-INF folder in your jar file to overwrite the old one.
6. That's it! Now install the jar to your phone and enjoy your full screen game...
EDIT: Btw, full credit goes to skopi (from another forum) for posting this info. I just tried to make it a little easier for everyone by posting my own guide including pics.