Arm bars, when done correctly, can work on even the best wrestlers. You'll have your arm over top of the bottom guy's, which will then feed through and over his back. Be careful not to take your hand toward his shoulder. That is called a chicken wing and the referee will stop it due to it being potentially dangerous. You want to keep your arm straight across his back.
There are a few ways to get an arm bar in, but when you have one, be sure to apply as much pressure forward as humanly possible. You need to keep your opponent's head on the mat so he can't do anything tricky to defend.
Also make sure you keep a good tight waist while you're securing your hold. If your opponent gets his hips away from you, you'll lose your grip and he may get an escape. If he does start to circle away, jump to the other side and look to dig in a half nelson so you can keep control with the arm bar. This also opens the door to use the half in a variety of ways.