The first step, as Pedro said, is to make sure that the headlight bulb is actually lit and you're not seeing the parking bulb, which is a dimmer bulb. After that...
The heat of the headlight can cause the power or ground wires to weaken and fray, resulting in reduced power or intermittent connections. I would pull out the light, pop off the back cap, and take a look at the wiring to see if there is a visible issue.
Another common problem (which usually results in flickering or the headlight not lighting) is that the mount for the electrical plug on the car side is not solid so if it gets bent at all then the headlight may not make a good connection. You can pull back the wheel well liner and hold the plug with one hand while pushing the headlight in to get a good connection. You can also lube the rubber surround on the plug so that it slides together better.