There are several cross genre tunes out there. The first that springs to mind is dirtyphonics - Polygon. This goes from 175 - 128 (or thereabouts) So making it smooth to transition into some dirty electro / dubstep or something.
Otherwise, if youre playing a bit more laid back tunes; use what the old hands use! The turntables! Turn off power, wind down deck, bring in new tune.
Another way is remixes, if you have a tune thats been remixed in the genre you want to switch to then mash up the remixes.