If you have spilt it or it has leaked onto the friction material on either the brake shoe or caliper pad, you should really replace the pads or shoes (in complete sets, as in both sides of the same end). The brake fluid will affect the friction characteristics of the material in a very inconsistent and discomforting fashion. As in, one moment that side will grab (pulling the car over to that side) and the next it will fail to grab, sending the car hurtling over to the side that has the clean brake pads.
That being said, if it is a VERY small amount that has touched the pads or shoes, brake cleaner can sometimes clean it off. But I repeat, you should REALLY replace them. The cost of the brake pads are minuscule compared to the cost of replacing you car. Or your life.
Off the soap box now....