I hitchhiked a couple of times. Once my friend and me went backpacking in Sierras from Sonora to Ebbets pass. It's around 40 or so miles by trail, one way. We planned that we would hitchhike back to the point where we started (around 125 miles or so by roads). We only had 3 days for the trip, so we couldn't do it there and back by foot.
The key was to look nice and presentable. And although we were dog-tired from the trip, not shaven, etc., we did have a spare fresh t-shirt just for this, and we did our best to clean up.
Ten minutes into the waiting at the Ebbets pass turn-off to camp parking, two girls in RV pull over and tell us, they'll give us a ride in a couple of hours, unless someone picks us up earlier (they were just heading out on a trail)
We wait about 20 more minutes, and Toyota Sequoia, fully loaded with bags and camping gear, pulls over. The guy behind the wheel asks whether our car is broken and we need a ride to the nearest pay-phone for tow-service. We honestly tell him we'd be happy if he could give us a ride as far as it worked for him & us, but since he is so fully packed, we are sorry to have flagged him down, and don't want to waste his time.
Well, lo and behold: the guy, his wife, his three kids (from teenager to a little girl) get out and repack their stuff, so that we could fit our 2 backpacks in the back. Then they squeeze on the seats and make room for us. The guy gave us a ride for almost 80 miles or so, to the intersection where he continued to Nevada, and we hitched another car up to Sonora pass (California side). He gave us his card: he had an auto-shop in Las Vegas.
The second car stopped after about 40 minutes. The second guy was a paraplane flyer. He was coming back from a long weekend with all the gear in the car, and had only 1 seat available. So my friend went up with him, and then came back with the car to pick me up. While I was waiting for him, I witnessed another interesting incident, but that's another story.
Overall, we were pretty lucky: traffic in that region is fairly light, and we could've waited much much longer. But if you are nice, and greet people with a smile and honest upfront attitude, you can get a long way.