Don't Do It

Some beginner riders constantly watch the road behind them, or fixate on their rearview mirrors, as if expecting Interpol to be chasing them. This comes at the expense of watching what's ahead, and usually stems from the falsehood that General Motors owns the highway and a mere bicycle can only trespass upon it if it dives onto the shoulder whenever an overtaking car approaches. A better idea: buy a LED blinker, and don't sweat it. A better tactic would be to spend all that effort in riding predictably so cars can pass more easily.
Do It

On the other hand, keep track of what's happening on the road. If you hear an engine, take a quick look back both to see if it's some pimply jerk in a Camero or an 18-wheeler hauling an ICBM. Looking back also lets the overtaking driver know that you are aware of him, and makes him more likely to treat you as a breathing human being who needs some space, rather than some road obstacle to freely graze. Look back to keep other road users from passing when it's not safe. If the road is windy or there's a hill, take a quick peek to see if you need to claim your lane until the dashed line reappears. You can't take possession of your lane to avoid being squeezed off the road unless you know if something's behind you. Besides; it's relaxing to know you've got the whole stretch of road to yourself.

Don't Turn!

When most riders crane their head to look behind them, their bike drifts in the direction they look. Some of the best bikers in the region have caused pileups on rides because they let their bikes turn when glancing back, and it's easy to drift right into oncoming traffic if you take a good hard stare at something.

The solution is to drop your left hand from the bars if you look over your left shoulder. In general, just be conscious of what will happen when you turn your head, and make the glance as quick as possible.
Practice

A great exercise is to have a friend ride behind you, while you glance behind you and try to count the number of fingers he is holding up. This is great for developing the ability to very quickly tell exactly what is behind you. Practice clocking the speed of cars behind you as quickly and accurately as possible, then look again to see if you were right. Learn to pay attention to detail, and use these skills once you have them.

Rearview Mirrors

While a rierviow mirror doesn't save you from looking behind, it's great at reducing the frequency of what is fundamentally a disruption in your riding and aerodynamics. However, don't think it is a replacement for looking back, because every mirror has blind spots, and bike mirrors have terrible ones.

Riding in Formation

When you've got a buddy sucking your back wheel, looking back every ten seconds to make sure you haven't dropped him is a real pain in the butt. A better solution is to use a rearview mirror, or to simply communicate. If your pace is too fast to be sustainable and your friend is dropping off, people should say so instead of spending the next hundred meters silently sprinting to get back out of the wind (which is what they do if you don't keep an eye out).