Stereo delays work wonders
Fills up the sound space whilst keeping your mono instruments relatively mono.

Generally I try keep noises fairly mono, and fill up the space using delays, reverbs, and widening on FX channels and verbs.

Make sure whatever you do sounds good in mono. No point making a kick ass synth part if it sounds weedy and lifeless when its summed down into mono. If you make party music, needs to be suitable for mono rigs as steve said....

There's a nice free stereo widening plugin i got called stereoizer thats mono friendly, highly recommend it.