Lets go over some simple things to prep you for your dyno, street, email tune. These are some stuff we notice time and time again. If you don't know or need advice talk to your tuner or some one at the shop tuning the car before hand.
1. Spark Plugs/Wires, Make sure you have quality fresh plugs and they are gaped to where you need them. Gaped to your Naturally Aspirated, Nitrous or Boosted Application. Also make sure have new wires that support your setup.
2. Fuel, make sure you have enough fuel to run your vehicle on during the time needed. If you are being tuned for E85 check with tuner to confirm what he wants in the system and how much of said fuel. Some prefer to show up on 93 octane first even if its just a retune.
3. Boost leaks, make sure you have double checked your application for boost leaks. Yes most tuners will recheck but this will save you time and money if they are not fixing leaks for you.
4. Nitrous, ensure you have plenty of nitrous in your bottle or bottles for the duration of the tuning. Also make sure every thing is in working order.
5. Fuel System, make sure your fuel system will support your build. You don't want to be running 46lb injectors trying to make 17lbs of boost on a built LS/LT etc. This goes for fuel pump as well. Stock fuel pumps run out real fast when going to boosted or upping boost on already boosted cars.
6. Mechanical, do everything in your power to make sure your car is in perfect working order before scheduling your appointment. This is includes tires, don't have worn tires that are ready to pop. Be advised also dyno's wear out drag radials and slicks and can provide lower numbers.
7. Time, make sure if doing either a street tune or dyno tune you have time. The simplest tune can some times take longer than normal. Plan to be there for least 2-3 hours. If things fail or need repaired because you didn't ensure the things above, that time could become 5-7 hours. Or the tuner may decide to just tell you to fix it and bring schedule another appointment.