The plaza is where people hang out most if you're looking for other travelers. Cross 101 on 7th street to get to a wooded area where you can urban camp.
East towards Weaverville/Redding
To leave Arcata going east to Weaverville/Redding on State Route 299, you can catch rides at the 101 North exit just at the north edge of town, 17th and G St, right at the pedestrian bridge to Humboldt State. Make sure to ask the driver if they're taking State Route 299 because if you don't, they won't make the turn-off for the 101 about 3 miles out of town and you'll be stuck trying to hitch a ride at the interchange, with cars flying by at 65 mph (not likely they'll stop here!)
Coming from San Francisco
It's easy to hitchhike to Arcata, California! Starting from 101 in Golden Gate, San Francisco there are some great spots to pick up rides all along 101. Try to get out of the Bay Area quick if you can as the California Highway Patrol will be on you if you attempt to hitch on the three lane highways. You shouldn't have many problems on two lanes roads. The farther North you go the easier it gets! Most people can recommend good spots. In Ukiah, 101 turns back into a freeway. Hopland is a great spot to hitch, especially on the straight run near the Solar Living Center! Laytonville is a great spot!