Yes, it’s great that modern DAW systems allow you virtually unlimited tracks, and you can do multiple vocal takes, guitar solos, etc., and sort it all out later. However, sorting out takes and comping together a final part is usually NOT part of the mixing process. This should be done before sending your tracks to get mixed. Be sure to send the best performance! Not a set of tracks from where the engineer must pick one.
Clean them up. Remove any background noises that you do not want to have in your final mix. All these little things that may not be heard before mixing will most likely show up in the master after loads of compression has been added. Make sure that all instruments and vocals are synched, that they follow the tempo of the song (specially the drums and the bass guitar). If some tuning is needed, this is the stage to do it in.
If you added some effects (like compression, EQ, delay or reverb) to your tracks, please remove them before sending them. Unless those effects are part of the fundamental sound of the track. In that case send two versions, with and without effect. If you used a Limiter, please remove it.
All files and tracks need to be labelled with useful and meaningful names so that the mixing engineer can quickly figure out what is what. Use proper names for each track, and try to stick to common practices as much as possible. For example, a typical rock drum track layout may include tracks like Kick IN, Kick OUT, Snare TOP, Snare Bottom, OH-L, OH-R, ElGuitar_solo, Bass etc. Try to stick with names that are common and make it absolutely clear what they are. Abbreviations are fine if they are commonly used and well known, such as OH for overheads, SN for snare, VOX for vocal/voice, etc. Properly naming your tracks will save your mixing engineer much time and frustration!
The final step in preparing your tracks is to consolidate and export your tracks. You need to send a separate audio file for each track, and ensure that all of those audio files all start at the same point in time (time zero, for example), so that the mixing engineer can simply import all the tracks into his software, line them all up with the same start time, and everything will play back with the proper timing. The file format should be as follows: 24 bit, 44100hz, WAV (16bit accepted).