1. Use a Koll or other type of balancer to make sure the blades are balanced and have matchingh CGs

2. Have you balanced the flybar?

3. Have you checked the main blade tracking?

4. Is your head speed too low (you should be hovering at about 4.5 - 5.5 degrees of pitch)

5. High gyro gain will cause the tail to wag but it shouldn't cause a violent vibration.

6. Are all of your servos (esp. the collective) nice and tight in their mounts?

7. What kind of training gear and how heavy is it?

8. The trims are right if the heli has no consistent tendency to drift in any particular direction from a stable hover in no wind. The only way to know is to fly it and take note of what it's doing. If the heli stays put for a moment when you take your hands off of the sticks from a nice flat hover, then begins to drift in different directions each time you try it, then you are ok. If it always drifts forward, then you need some aft cyclic trim, etc.