Check the brake cables for fraying or rust. If either is present, replace. Check the outer housing, to make sure there are no kinks, or any other damage. Bliss Images Always double-check the quick release skewers. While there are many different adjustments, and many possibilities for problems with your drivetrain, a quick check and a couple of tips will help you avoid most common problems.
Any loose part can diminish shifting performance.
After tightening everything, check the chain. It should be slightly oily to the touch, not dry, and not dripping lube. There should be no rust, no kinks or stiff links. Depending on how much you ride, the chain should be replaced every 1000 miles or so to prevent excess wear to the cogset and chainrings, and to reduce the chances of the chain breaking. Check to see if the chain is too long or too short.