Do you pack a toiletry kit for yourself when you travel? Why not pack one for your pet as well?
If you're traveling out of state, bring your pet's health records along. You'll need them in case your pet bites someone or gets bitten. Put your name, destination address and cell phone number on your pet's crate in case of emergency. Also, don' t forget small first-aid items like nail clippers and liquid bandages.
Portable water bowls -- you can even buy one that folds up -- and bottled water are a great idea. You never know what might upset your pet's tummy, and you certainly don't want to find out while you're trapped in a small car together. So, stick with water -- no food.
Don't forget a pooper scooper, waste bags, treats, an extra leash and your pet's favorite pillow or toy for comfort.