1. If you know that a class variable will not change, make it "static final".
2. Use as few methods as possible.
3. Use as few classes as possible.
4. Use method instance variables over class instance variables. Note: Jury is still out on this for me. I've seen a reduction using this technique, but not sure if it was truly the idea or the obfuscator made the difference.
5. Use something like ProGuard to further obfuscate and reduce your code.
Other? I am relatively new to J2ME development, too, so I am still learning the tricks of the trade as I go along.