Pre-loading the fork

I recently picked up an 04 CRF150F for my son. He's a big kid, around 170lbs, and the front forks on the CRF sag quite a bit under the bikes own weight and especially so with him on it. I've heard that you can pre-load the fork springs by adding washers or PVC tubing to get the proper amount of sag. Is this an ok practice or are stiffer fork springs my only option? This bike uses the old style conventional forks.

Thanks,

sci85

That's what I figured. Thanks.