I normally create a concept mix in a single night to gauge if my idea is viable/doable, I then spend the rest of the week slowly refining and evolving the track. This process usually equates to adding moar noises, but on a more serious note involves changing things around that become boring to me during that period, to make things more interesting for the audience (including fills, breakdowns etc).
The mixes here in WCRG2013 have taken a little longer as I've been exploring genres I don't normally touch, so that has involved some research each day to get a feel for the style, and for these vocal mixes - to get an idea how I want to sing the thing. While on the topic of vocals, these are written on the fly shortly before I perform them, they tend to write themselves. Impromptu vocals when the urge hits at 2am in the morning usually brings stellar results.
I should mention that this whole process from start to finish is usually interluded with bouts of laughter.