You can declare things that require a script (valiums, steriods, ect), but you have to get a script in Mexico. And it has to be a certain amount (for personal use). A friend of mind does it all the time with valiums. You can only bring back like 40 pills per person though.
The catch is once your in the US, police can pull you over and confiscate your meds and write a ticket. The Mexican script isn't valid.