Don't drive your own car...
It may get stolen across the border, and getting it back is a HUGE pain.
Be prepared to be solicited often by little children selling Gum.
Be prepared to spend a lot of money on tourist traps if you don't speak Spanish.
Go with someone who has been there before and speaks Spanish.
Hide some money somewhere where you would normally not have it. Pickpockets abound in Tiajuana.