Check the Chain Tension
Most chain drive openers suggest you tighten the chain so there's about 1/4 in. to 1/2 in. of slack from the rail to the chain (check your manual). Over tightening the chain will put excess wear and tear on the shaft and gears. Too little tension could cause the chain to skip off the sprocket and fall down on your car.
Adjust the Travel of the Door
There are two knobs on the opener that control how far the door travels up and down. Your door should press snugly against the garage floor so the weather stripping seals the gap. If the door travels too far down, it will cause the rail to bow upward. This also causes excess wear and tear on the shaft and gears.
Your door should travel up far enough so that the bottom of the door is just about the same height as the doorjamb. There is a door stop that prevents the carriage trolley from crashing into the opener. Make sure the trolley stops before hitting this bolt.