How to prevent? Depends on whether you are using a trike gear or tail dragger. On a tail dragger, the main gear must be slightly ahead of the CG (usually right under the leading edge of the wing will work) and; you should have some toe-in on the wheels. The closer the wheels are to being under the CG, the more prone to noseovers but; if you get the gear to far forward you will have a great difficulty in landing without excessive bouncing. A compromise is mandatory.
If you are using a trike gear, nose overs are usually caused by excessive movement of the nose gear or springy gear--you only want about 5 to 10 degrees at full deflection. Toein also helps prevent groundloops on a trike gear.