A good pair of PANTS!
Okay a good pair of slacks is a necessity, and it has to FIT! Not too tight so that you feel like your going to bust out of your seams if you sit down and not too loose or long that you're going to trip over yourself. If you have curves get some with a little bit of stretch but remember not too tight!
The Gap is one of the best places to buy great fitting pants for a low price.
