Depends... I find I (frustratingly) compose entire tracks in my head as I'm walking home, only to forget them as I wait for my PC to boot :) I've taken to attempting to record myself attempting to vocalise what I'm hearing in my mind into my MD recorder as I walk.

If it's pure techno I'm creating then there's nothing like loading a whole bunch of totally random samples into a soft sampler and writing some random funky rhythm. (Aforementioned mouth noises, suitably pitch modified work nicely).

Otherwise I have a Korg MS2000 which I fiddle about with for a while and, if something good emerges, record that.

Perhaps the best thing I've learnt this far is that everything sounds crappy on its own, so I should just add stuff until there's enough layers for some interactions to start happening, then pick up on the interactions and attempt to bring them out, gradually stripping out the original layers. Or not, as the case may be :)