If there is too much weight coming down on your garage door header, it will start to sag. Not taking care of the problem as soon as you notice it could cost you more money in the long run.
Before You Start
A sagging garage header is, essentially, a structural engineering problem. While the most thorough and sure way to eliminate the problem is by getting a structural engineer in to look at the garage, take measurements and fix the sag; there are things you can do yourself to fix the problem, although they may not be as permanent as having a professional fix it.
To correct a sag in your garage door header, you will need to reinforce it. If you take the garage door measurements to a local lumberyard or hardware store, they will be able to help you pick out the proper support beam for your situation. You can go with a wooden brace, but depending on the load on your garage door header, the sag could reoccur overtime. By using a steel beam, there is less chance of a re-occurrence in the sag.
To install the extra header, jack your door up to correct the sag and bolt in the beam.