Excellent singletrack can be reached by riding along the Forest Service Roads. Just look for the signs posted along the road. Expecially good is the Preston RR trail. It requires determination to get to it, 3.5 miles up hill, but definitely worth it.
Travel West on Hwy 18 from I-5, or East on Hwy 18 from I-90. There is a gravel parking lot on the North side of the hwy at the crest of Tiger Mountain. It is apporimately 4 miles from the junction of I-90 and Hwy 18, or about 20 miles from the junction of I-5 and Hwy 18.
http://www.singletracks.com/item.php?c=1&i=545