From I-5, take the Lake Siskiyou/Mt. Shasta City exit, and follow the signs towards the lake. Shortly after crossing the Dam, take the left up to Castle Lake. This viewpoint is about 5-6 miles up the well-plowed road. Most people do not see Shasta like this. I never had before my last trip.
This lookout point on the road up to Castle Lake provides an epic view of Shasta's vertical rise. This photo was taken in February 2013. The blue and pink colors in the alpine winter sky above the mighty mountain only lasted for a minute, but I was glad I waited before heading back to town.