Always know about the track you are going to mix in, whether it is too fast or too slow. You don't want to adjust the playing track (should go without saying) Often I would start the track I'm bringing in at what I think is a little faster than it needs to be. Then I would know it was too fast and would tweek the speed down until it matched. If you know your tracks you can be pretty accurate with this. Another thing is to listen for uniqueness in the beats, for instance the track you bring in may have a cymbal every beat that the playing one does not therefore you can hear whether that cymbal is ahead of the kick or before it.