Even the tiniest problem, such as a broken taillight, can give an officer a legal reason to stop you. Don't trick out your car: custom lighting, too dark tint, and loud mufflers are common reasons for a stop.
Watch your speed-- and the speed limit
Many roads and highways have speed limits that change several times. This is where the cops like to hang out! Once you pass a speed limit sign, check your speed and adjust it to the posted speed: it's the speed you should be traveling, not just slowing down to.
Watch the time
The cops are looking for speeders and other traffic offenders when they're most likely to find them. This means weekend nights, game days, holidays (even St. Paddy's!) or just right after pub "closing time" is when you'll find police looking for drivers under the influence.
Drive defensively, never aggressively
It safer for us all, plus cops tend to pull the drivers who stand out from the pack.