1. When recording your drums record single hit samples of all the drums but mainly the snare, kick and toms. The reason for this is that you have some good samples that you can feed in when needed. Generally speaking this is invaluable for toms that were played when the cymbals and/or ride were blaring in the mic and a clean signal isn't possible. With the samples you can just drop them in place of the noisy tom hits for a cleaner sound.
2. When recording your guitars use a DI to split the signal so you can record the direct signal from your guitar as well as the amp signal. This one is really crucial because if your mix engineer wants to try out some new guitar sounds by re-amping or running the DI signal through an amp simulator plugin. When you give your mix engineer this the possibilities are endless! I personally use the Radial JDI MK3 Passive DI. You can pick one up here:
3. Another help is recording the midi output from any keyboards in addition to the audio output. Most sounds are internal in the computer now anyway but if they're not it's great to have access to midi files to change sounds at the mixers discretion. This is especially important for the more urban/keyboard based songs with electronic drums. It's always great getting the midi files to replace or supplement the original sounds.
4. Last but not least is labelling and organizing your tracks neatly so your mix engineer isn't pulling his (sometimes sparse!) hair out trying to figure out which track is which. It's amazing how many times I've mixed a track only to find out that I used the wrong guitar tracks as the "main" tracks because they weren't labelled properly. Here's a FAQ I made on how to do this: http://www.morphproductions.com/toronto-recording-studio_faq.html#mixtips
If you know you're going to use a mix engineer to finish off your project it wouldn't hurt to contact them in advance for any other tips or preferences they may have. As with anything else planning is key!
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