belts are always a good accessory to top off whatever you're wearing. black is a great default colour, as it looks pretty sweet on anything from jeans to khakis. white is a good colour if you're wearing darker colours. word of caution, do not wear a belt that matches your shirt colour (with the exception of black), it looks terrible.
on a related note, if you insist on tucking in your shirt, please wear a belt. also, stay away from large, obnoxious buckles... unless they serve some purpose, like wearing the Batman logo to some convention (okay, bad example).
so, go pick up a belt. or three. they're cheap in price and a lot to the way you present yourself.