INTRODUCTION
There's something very exciting about recording outside the safety and comfort of your studio. It's like an adventurous journey for a producer, a one that not only offers unusual and interesting soundscapes, but also one that presents many different challenges.
GET AS MUCH INFO AS POSSIBLE
Knowing where you will have to set up your recording camp is the first and most important thing, as it will generally give you an idea of how things might sound and what to expect.
Given that the range of places you may go (from recording in a small wooden cabin in the countryside to tracking drums in a church or even recording in a wide-open space under the clear skies), the tools that you will need may vary greatly.
Also, get info on not only how the place will sound, but also what other sounds may there be in the background - this is very important to avoid any surprises like recording next to a very loud airport or things of similar fashion.
Make a list of potential problems that you might face during the recording process and talk them over with the client, so that everybody is on the same page and everybody is aware of the potential issues.
GET YOUR CABLES READY
Once you know where you're going to record, be smart and create a mic input list to know how many mics and cables you will need. This will not only help you pack and organize everything, it will also tell you how many cables you have to bring with you.
If you're recording in a huge space (for example, a church or a huge hall), be aware that you will probably want to record room tracks to sonically utilize the available space as much as possible.
Room tracks are usually placed further away from the source to capture the ambience, so the cables have to be long enough to allow you to do just that. It's a good idea to bring more cables than you think you might need, as well as making sure that the cables are longer than you think you may need. It would really suck to find out that you are a little short on cable length when you are setting up, especially if you're in a remote location that does not have a music store nearby.
If you have a multi-core stage box or two, take those with you - they will significantly reduce the need for long individual cables and will make routing vastly easier.
SET UP A CONTROL ROOM
Not always will you get a chance to set up a separate control room, so you might need to prepare for tracking in the same room where the musicians are playing. This is highly inconvenient for monitoring purposes as you will have to monitor the tones by playing back the recorded material, but sometimes you will have to work within the boundaries that are there.
In order to be able to monitor when the musicians are playing, you will need to have a pair of good headphones. If you can, take your studio monitors with you as well, so that everybody can hear what they recorded once the take or session is done.
Also make sure that you can provide the necessary monitoring options - long cables, closed back or in-ear headphones and a powerful headphone amp is a must. Always be ready for even the stupidest of situations and problems.
BRING YOUR TOOLBOX
You can never be too safe when it comes to recording. Even if you have enough cables, some of them might die in the process. Even if everything you have is new and in pristine condition, things can go wrong, especially when you least expect them to. You may need to fix up a drum kit that has loose screws from transporting it, or you may have to fix guitar intonation issues or even replace dead pots and switches. There are a million things that you won't even suspect to go wrong.
Make sure that the toolbox contains a soldering iron with all the accessories, duct tape, painter's tape, cable ties, a few sets of different keys and wrenches, spare cable connectors, a knife, scissors and everything else that you can imagine. A toolbox is never too full.
When things go south, you'll be the hero that can quickly save the day with all the tools that the musicians have not thought to bring. This will save time and money for everybody.
BRING TWO OF EVERYTHING AND BE OVERPREPARED
This is literally one of the most important points. Have backup for everything if you're going somewhere without a music store. Without you knowing, a bad power supply can make some things act weird or not function at all. In this case a spare unit might save your back. Make sure that the computer has a back up power source like a UPS unit or a good battery if it's a laptop. A power conditioner can also save you from horrible electrical hum that you may sometimes get from a bad power line.
Take extra things with you like acoustic panels to kill the most annoying reflections or to separate instruments that are bleeding into each other too much. The panels may also help you set up a more usable listening spot. Extra microphone stands can also come in handy, as well as extra hard drives and power extension cords - simple things that might save you.
Keep everything organized and stored in safe cases to avoid damage. If you break your gear, you are not really working with a profit.
Also keep notes of everything you've packed for the recording so that you can cross things off the list once the session is done to make sure that nothing gets lost.
CONCLUSION
In short, you have to be ready for everything, even for the things that you are not really aware of. Seems a little out there, but you can never really be too prepared. But you can do many things to minimize risks.