It really depends on the track. If I want it to sound FULL, then there's a lot of stuff that goes into it. If you don't mind
a quick shameless plug, around two minutes in, this is what's playing:
That buzzy lead (I like buzzy leads like that because they fill up a lot of high frequencies and sound big.)
A subdued lead harmonizing with the main lead
A stabby synth playing a countermelody
The piano, playing both the melody, harmonized melody, and chords
Another stabby synth doubling the piano chords
Drums (Kick, snare, and hi-hats with plenty of delay on them)
Sub bass
Pads
Filtered white noise sweeps
So it's partially in the arrangement and partially in what sounds you use. The easiest thing to do would be to use a fuller-sounding lead, busier drums, a sub bass, pads, and white noise. The best thing to do, however, is introduce harmonies and countermelodies. They don't even have to be prominent -- just there to make the whole thing sound busier and fill up empty space, if you have any. And if you can make that countermelody play off the main melody so it changes notes when the main melody is standing still, you'll have something that sounds a lot fuller than you'd otherwise have.
Doubling important parts with other instruments is also fun, though then you have to be sure to EQ them correctly so they don't fight for the same sonic space.
Effects-wise, chorus is fun, as are stereo wideners. One thing I did with my pads is I duplicated the track, hard-panned them left and right, and then EQ'd the two tracks differently. For things that have more definite attacks, you can do the same thing, but introduce an offset between the two, so one triggers a little after the other. That'll widen the stereo presence while making it sound fuller.