I think the most you can do is develop portable code that is coherent with the JTWI specs (better read it) and test it as much as you can in as many devices of interest as possible.
Basically JTWI devices devices with mandatory MIDP 2.0+ WMA 1.1 + CLDC 1.0 support and optional MMAPI 1.1 support, at least. WMA 2.0 and CLDC 1.1 can also be enabled instead since they are supersets of WMA 1.1/CLDC 1.0.
JTWI also enforces some requirements which are optional or do not exist in the original MIDP, WMA, or MMAPI specs. For example, in regard to MIDP, some requirements on RMS; screen resolution,JPEG image format support etc are defined by JTWI. [ March 21, 2007: Message edited by: Eduardo Marques ]