How can I make my sink smell better?
Does it smell like sewer gas?

If so, the sink trap is probably incorrectly installed, or more likely, the drain on an adjacent sink or dishwasher isn't properly vented.

A "vent" is a portion of drain tube that lets air into the pipe to replace the water flowing downward.  If there is no easy way to draw the air into the pipe (usually the vent just goes up to the roof and draws air from outside) then the water from other nearby traps can be suctioned down the drain as well.  Without water in trap, sewer gas is free to come up the drain and make you want to move to a condo.

To check the problem, run the dishwasher or wait until the stink appears.  Then get under the sink (with a bucket!) and take off the trap for the sink that smells.  This can usually be done without tools since it's usually a threaded plastic pipe.  Check that the trap is full.

If it's completely full than all this advice is moot: listen to some other commenter.

If it's not full, then the best thing to do is to fix the drain properly.  If that's too difficult or too expensive, you can install a "cheater valve" right before the trap.  They're officially called "air admittance valves" but nobody will know what you mean if you say that. :) They are one-way air valves that make it easier for an improperly vented drain to draw air through THEM rather than sucking out the water from the trap.  They're often used in island sinks where it's impossible to get a proper vent.

(source: homerepairforum.com)