mixing between different BPM
how to mix between different BPM ? example from 90bpm to a 128bpm
Hit the sync button 
On a serious note, though, you should first consider if those tracks are really supposed to be mixed together, 90 and 128 track have a different vibe.
In case they do, you might need to build a a transition that progressivly goes from 90 to 128 ( or from 128 to 90 ).
Don’t know what music style you’re playing, sometime a good speech makes the cut.
transitions between different genres/speeds
You using Traktor? Check out this post by Ean:
You can also do this with different software provided they also have the same effects. 
Experiment
There are lots of ways to do this depending on what your setup is. Could we get a little more info on your gear? If you are using digital software to DJ which I can almost guarantee you are play around with it. You can find a third track and make it into a transition track. You can utilize effects like delay freeze echo all sorts of stuff. Are there 2 tracks in particular you are working with or are you just looking for techniques in general?