Reply to Mixing two tracks in different bpm without effects

I have a problem. I'm playing a house/disco/funk set here in two weeks. Problem is that some of the tracks I'm playing seems to be recorded by an orchestra thus having a fluctuating bpm. I seriously don't know how to mix from one to another without the use of efx AND make it sounds good. I'm gonna be using a cdj and a mixer with no effects. Any ideas?