If your next vacation involves renting a car, then you've got two choices: Bring your own car seat, or rent one along with the vehicle for a small fee.
Renting: The upside to renting is convenience -- plus the rental car company may have employees specially trained to install their seats. The downside is that you can't be sure of the seat's condition or comfort level, since some companies take better care of their seats than others. When you make your car reservation, make sure to indicate whether you require an infant seat, child seat, or booster seat, notes Diane Karl, with Avis Rent A Car. If you wait, you may not get one at all.
Bringing your own: If you bring your own seat, make sure you also bring the instructions it came with and ask to see the owner's manual for your rental car for additional installation info. You might also want to ask if the car is equipped with LATCH -- Lower Anchors and Tethers for Children. This system is the new government-standard hardware for installing safety seats. It is required in all new cars as of September 2002, and it's worth asking for a LATCH-equipped rental car to increase your chances of proper seat installation.
Finally, be mindful that child-passenger safety laws vary from state to state. New Jersey, for example, amended its laws in December 2001, requiring that children up to 8 years or 80 pounds ride in a safety or booster seat, while New York only requires safety seats for kids younger than 4 years old. Better yet, take a cue from the stricter states and adopt their safety policy permanently. It could save your child's life.