Play with your edit functions. Slide them over a few ticks. Transpose a couple of instruments up or down an octave, and listen to the result in relation to the whole song, not just on its own. Try quantizing things in triplets for a few measures. Scale those velocities section by section to give the piece variety in dynamics.
Nothing beats creativity.
This is all non-destructive stuff. If all of your sound cards or external modules are made by the same manufacturer, you may notice that the sounds are all equalized in a similar manner. If you have the luxury, try mixing instruments from different modules within the same song. If you're routing your instruments through a mixer, try notching them differently. Each instrument's EQ should be finalized only in relation to everything else.