Some weeks I'll have a rhythm or melody in mind that I can work out on an instrument and use to begin a piece, but other weeks...there's not a lot in mind. In those cases, I'll pick something arbitrary and begin: maybe come up with a little syncopated rhythm, and get that down on a MIDI drum machine. Or maybe just a tempo that feels right, and play bass or guitar against it, or some similar approach.

I find that I don't often spend a lot of time looking for "sounds" anymore--I might start with a preset and tweak it, or leave it alone...or start with the plain sound of the instrument, get the notes down, and tweak sound later. It's easy to get lost in "finding a sound so I have an idea," but I find I'm more productive if I don't look for ideas in the timbre of whatever instruments I'm using.