I had about 140Kb of jar size and I compressed it to 58 Kb.
What I used and I recommend to everyone:
1) put all resource files into one data file - the major optimization.
2) optimize palette for your graphics files. It would be better if it will be the same and 256 colors.
3) combine small graphics files onto one big file.
4) obfuscate the class files.
5) reduce amount of methods in your classes.
6) reduce amount of classes.
Small tips: do not use set/get functions for variables. It takes pretty much space and you actually won't make your code much better. If you have some variable in the class that is used by other classes, make it static.