on trance I focus on riffs or melodies and then build on them.. my more 'successful' songs have been ones where I had the idea in mind before I sat down.. though I've done some nice stuff by just ploinking along until I got something I was happy with..
on more drum-complex stuff I focus (naturally) on the rhythm of the drums.. usually I get a rhythm going with my kick drum(s) then I tap in the snare or handclaps...
sometimes I do it the other way around.. especially if I do sets of 16 in 4/4
either way/style I use a lot of creativity.. with a lot of focus on movement.. does it move or does it sit still.. and does it make me bop my head to the beat or not.. if not then I either try to fix it or scrap and move on.
thats about it.
oh, and I'm usually tons more creative after doing something school related (highly technical and non-musical) especially after a few days/week of no music making ...then the ideas just flow... its like my brain is fully awake and recharged then.
if you're having problems coming up with something then it means just that.. you're forcing yourself.. like all things good, music shouldnt be forceful.. it should be more natural and easy to achieve... at least imo.
