If you are looking for a minimalist answer, I might not be the one to ask. On my current assignment, I loaded my full-sized truck with everything that would fit and then, just for good measure, added a 6×10 enclosed trailer full of stuff (including my motorcycle). You certainly want to take everything you think you will need, but I doubt that everyone will require as many toys as I do.
When first deciding what to take with you, try to anticipate how long you will be there. If you start an assignment in Northern California in the summer, you might get away with shorts and tee shirts; however, if you extend into the fall/winter, you will very likely require jeans and sweaters and possibly cold weather attire. The length of your stay can also affect the toys you might take. For example, if your assignment in Colorado starts in late spring, you might pack your tent and all your hiking gear. However, if you extend your assignment into the fall or winter, you might want to also pack your skis as well.
Once you have accounted for the time you might spend in any given location, consider where you will be staying. If you are staying with a friend or relatives, you might not need a lot. However, if you will be moving into an apartment, you need to consider even the smallest things like shower curtain rings.
When you first confirm your assignment, start making a list. Keep it somewhere handy so that you can add to it as you think of things. On your first few assignments, you will undoubtedly take too much. Eventually you will learn what things you cannot do without and what things you can leave behind to allow a little extra space in your car (to fit all the crap that you inevitably acquire while on assignment).
After your first few assignments you will fine tune your list and hopefully have one less concern when it comes time to pack up again.