eh? explain. wat u using now?
also, if i haven't driven this point home yet ^^; the small number of instruments DOES help everything stick together. I'd much much rather have a few, very good fitting instruments, than way too many that clutter a mix.
BUT it can -sometimes- be useful, to add some more layers into the mix, IF that's what it needs.
This is all very case dependent and it's possible that you don't need this at all. HOWEVER, you're probably right in thinking that it might be useful in some way to explore it a little.
Again, dragor's examples provide probably the best illustration of this. Not only does he do it with his own projects, but he points out examples of real professional trance tracks where you can hear the concept of "fullness" as he describes it.
some things to keep in mind:
-delays and reverb--they have their places! for example, an arpeggiated synth might not need delay since it's playing so many notes. However, a "chorder" or single-note "pluck" type synth might benefit greatly from a delay.
-layering! layering sounds can be very, very useful (not just for giant supersaws either ^^. Ex. adding more oomph to a bass synth, or adding a more pronounced initial pluck sound to a long sustained synth. Experimenting can be good, but in general it's better to have a reason for layering ^^;
-basses--"fullness" in basses is a bit of a different concept but is also very important. I use boobass all the time for this.
-Volume levels--elements that you add =solely= for "fullness" shouldn't be that loud. Should be juuust loud enough that if you're really listening FOR it, you should hear it, but shouldn't stick out in the mix enough that you would hear it with everything else playing.
-different kinds of melodies--dragor has something to say about this too--for example the difference between "gated" synths and "sustained" synths...
Of course, I can blather on all day about this stuff, but in the end it's REALLY just up to listening to things, trying new things, seeing what other people do, and in the end just deciding what sounds best/what you like.
Cheers,