Squamish is a city in Canada.
North towards Whistler
It's easiest to hitch just past the lights at the intersection of the 99 and Garibaldi way, past the pedestrian overpass. It's a long, straight stretch, traffic isn't moving too fast yet, and there is plenty of room to pull over.
South towards Vancouver
Right in front of the Stawamus Chief Campground, about 100 meters north of the pedestrian bridge, there's a public parking area that's preceded by a long, straight road. Hitch from the entrance of the parking so that prospect drivers can easily pull over in the parking.
If you're in downtown, you can catch bus number 5, get off at Stawamus Chief, and cross the pedestrian bridge. IMPORTANT: bus #5 apparently doesn't run year-round only in the summer.