I have done two small games for J2ME (DespullatMID,PetaBolas). You can find a shareware version in http://www.middletcentral.com
Before you start with J2ME, I recomend to play a little with java applets. The best way to develop a J2ME game is first to develop the applet's game and then make the translation to J2ME.
I should recomend not to use float operations in your game design because J2ME don't support them
Don't use MouseListener, it's better to use keyListener in this case. Instead of Panel try to use Canvas for your graphics, it will work faster and there is no Panel in J2ME.
Because the limited resources of movile phone don't use recursive algorithms because you can run out of stack and it consums to much CPU's time.
I know that there are a lot of limitations, but you have to think that your game could reach many, many, many people. Just imagine that your game is as succesful as Snake's game. Think about it.
I have to recomend you to download from http://www.middletcentral.com
a game called Arena v.1.1.0 from JavaXpert Ltd. It's awsome what someone can do with so few machine resources.
Well, I hope that this will help you a little.