Before setting out, make sure that you have certain essentials with you. Although you always hope for everything to go well, remember that you COULD find yourself in unfortunate circumstances, and it is better to be prepared.
Must-have's depend on you, and on where you are going.
If you are on any medication or supplements that are necessary to keep taking, be sure that you have enough for the period you plan to be traveling, plus an extra few weeks, if possible. Label everything clearly, and store it somewhere other than your purse or suitcase, but out of sight. If your money gets stolen, you don't want to desperately need medication and be unable to see a doctor or buy it. For the same reason, it can be a good idea to keep a copy of your prescriptions with you, or at least the information so the issuing pharmacy/doctor/optometrist, etc. can be contacted.
Speaking of having your money stolen, (which we hope doesn't happen!) divide it up and keep it in separate safe places, so that hopefully you won't lose it all. Better yet, use traveler's checks if possible, and keep the numbers and other information in a separate place. A great place to keep this information safe is in an email to yourself at an account you can check anywhere (like a yahoo or hotmail email account).
If you are traveling through very cold places where temperatures are below freezing, pack blankets and, if possible thermal-backed insulating blankets also, for each person. If you should have car trouble in the freezing cold, you want to be able to stay warm until help arrives.
In the same manner, if you are traveling across a dry, hot, hours-long stretch, make sure you have plenty of water stored. If your car breaks down, you don't want to be out there for hours with nothing to drink.
Think along these lines before you leave, and be prepared ... hopefully you won't need these things, but if you do, you'll be glad you did.