Properly sending the files to your engineer can go a-long way for the final mix.
For starters, you should send the engineer a rough mix or reference mix of the song(s). This way the engineer can keep the mix in the wheel house of your desired outcome.
If there are certain effects in your reference mix you really like, be sure to note them for the engineer and if possible send a snap- shot of the plug-in and settings you've used. The engineer will include this in the final mix for you with some serious precision.
Properly name all of the tracks in the session and remove any unwanted tracks. Disable any and all plug-ins before exporting the tracks individually out of the recorded session. By doing this, you give the engineer the flexibility to really finesse the final mix.
Export the tracks individually out of your recorded session into a folder you've made specifically for these files. Clearly label the folder and note where the folder is located. Once all the files are in this new folder (including reference mix and snap shots) compress the folder to a .zip file. Now you are ready to upload and send your files to the engineer.