Familiarity with brushing is the best way to get your pet to relax. Doing it in a quiet place, when you yourself can be relaxed, will really help. Sit them on your lap, or follow them around in a non-threatening way if they prefer, and let them sniff the brush to acquaint themselves with the object. Then work up from short, gentle strokes to long sweeping ones. Building up from short to longer sessions will also help, and a treat will make it a nice experience to remember for next time.