It is fine to include additional tracks you believe might help the mix, but these should be clearly labeled as alternates. Also, it is important to communicate to the mixer why you might want/need them.

Remove unwanted noises between performance parts. Sometimes it is OK to leave a vocalist's breath for emotion or a guitarist's or drummer's noises, but those should be communicated to the mixer. Solo all your tracks to ensure that no improper edits cause clicking, popping or other artifacts.

Most mixers do not include this service as part of the mix rates. If you need it, then be sure to discuss this in advance of the mixing session so that the additional work can be included as part of the project cost. It is best that this is done with using the tuning of the track to be mixed.

Disable effects and processors on each track of your session like EQ, Compression, Limiting, Reverbs, etc. In some cases, you may have created a special sound that you want to maintain. It may be a special timed delay that creates a rhythm that you have based your song around, or a very unique effect that is very important to the overall tone. In those cases, you may want to send the mixing engineer two versions of that part, one with the effects on and one with them turned off.

Sometimes the mix engineer will need to re-amp the guitar or bass tracks to create additional layers or textures within the mix. This process will allow for more impact or maybe fix a less-than-ideal guitar/ bass sound.

With all of your processing on, bounce a mix to include the total sound you are going for. Rough mixes help your mixer hear your emotional response to the song. This version also helps to show where the instruments are in the stereo field as well as which ones are of interest (louder) and which are more for just support (background).

Reference tracks with similar instrumentation and sounds to the mix you’re working on will help the mixer balance the groove vs. the vocal, the amount of low end, the height, width and depth of the song. You may like the sound of a band playing in a medium sized music club so you will pick a reference with the same room sound and same instruments you are using. It does not work too well if you choose an acoustic song and your song is a rock track.
For vocals you may want to have them up front with less or almost no reverb. Or maybe you want just the opposite. Choose a reference track that clearly demonstrates these qualities.
Sometimes you might hear a certain tonal quality of a particular instrument that you want yours to sound similar to. As long as you have recorded it to be close in sonic quality, then you have a chance of getting there.
The mistake many people make is in thinking you can change a sound into something it can never be just by mixing. Try to pick reference tracks that you think sound amazing and have had commercial success.

Short and simple is usually the best. Titles like Kick In, Kick Out, Snare Top, Lead Vocal or abbreviations like Kick O, Snare T, T 1, Vox (vocals), EG (electric guitar), RTM (rhythm) are common and help easily define what instrument the track is. Labeling your tracks clearly during recording will save you from having to rename mislabeled tracks later.

Ensure all tracks start from the beginning of a session and are continuous to the end of the parts contained in that track.
Track formats should be .wav including the bit-rate (24/32bit) and frequency it was recorded (44.4/48KHz)

If the song contains tempo changes, it is important to include these either with a midi file or a text file with marker locations and tempo

It is helpful to have a properly labeled folder that includes your song title, bpm and the key.
Example: Song Title_120bpm_Cmaj.
If there are many tracks (70+) it may be useful to group the different instrument groups into separate labeled subfolders within the master folder. This will save time when the mixing engineer imports them by groups such as drums and percussion, keys, lead vocals.
Compress the folder using .zip or .rar format to help reduce the overall file for sending

This might be one of the most important discussions to have before starting a mix. The way you expect something to sound is likely to be very different than someone else's view. Should the vocals be upfront, in your face or a bit more in the back of the soundscape? What elements are in the front or which ones need to be highlighted. Which are in the middle of the space and or very back? The more of these things you can communicate before mixing begins, the easier it will be to get closer to your vision of the final sound. Also specific musical samples will help your mixer understand what you are trying to achieve.