My answer assumes that you've taken some violin lessons; the action of the bow arm is very specific and complex, and I can't do it justice in text. There's just so much to teach. That being said,
You're probably not putting enough weight on the bow. The weight of your arm should be concentrated on the part of the bow beneath your index finger; this is only possible if your forearm, elbow, and wrist are aligned the way they're supposed to be. In addition, you want to increase the weight as you approach the end of the stroke. Don't force the pressure, just let the arm-wrist-hand unit rest more heavily on the bow, and it should do the trick.