Listen to what you are playnig .. the main instruction to this (I think) is to pratice practice PRACTICE .
Don't rely on you BPM Counter .. if you want to rely on it .. try the Reloop
RMX-30BPM .. A friend of mine has one and we mix with it verry often .. but remember this is just a helping hand .. listen to the first beat set the next beat in one line with the first and just play with the pitch .. keep aligning the beats until you got it right ..
Good luck