Nelson is a very welcoming city with a strong punk/hippy/vagabond community, and almost everyone is happy to chat and give advice.
Rides are usually plentiful and easy to get.
South towards Salmo
On the road to Shambhala? Hike up the hill south, out of the city, on highway 6. There is a rest area along Cottonwood Creek, where cars can pull in and park. This is THE spot for hitching out of town southwards. Guaranteed rides in no time.
North towards Kaslo
You want to cross the big orange bridge, and walk up a little ways. Its easy to find rides, just make sure there is space to pull over. Ainsworth hotsprings are this way, as are random beaches on Kootenay Lake.
You can also stand near Dairy Queen on the Nelson side of the bridge. Cars are going slow and there's plenty of spots to pull in. Everyone knows what direction you're heading.
East towards Castlegar
Stay along the water on Highway 3a, the Nelson Nelway. This will also get you into the Slocan Valley.
Places to avoid
Don't camp under the bridge or in the park near it. The RCMP have a station under the bridge and while people do manage it, more than likely you will get pinched. Cottonwood Falls is also bad for RCMP patrols.
Accommodation and Sleep
You can hike down the traintracks from Cottonwood Falls, west, and camp on the beach. This will get you chewed out and possibly fined if you are caught, as it is railroad property. Red Sands beach is also said to be good, on the east side of the lake, north on Bealby Rd.
Other useful info
Hot breakfast at the Salvation Army on Victoria St. and Ward. Great swimming in the lake.