I just returned from an "open jaw" hike in Yosemite. Entered Lukens Lake TH and exited Ten Lakes TH (permit issues). Needed a ride from Ten Lakes TH to Lukens Lake TH. It had just started to rain, which may have had something to do with it, but I had no problem getting a ride within 10 minutes. I did have many vehicles with one passenger and 4+ seats just keep passing by. I had one SUV with a (foreign (non-US)) family screech the brakes to turn around and offer a ride. "No empty seats" I said. "We can put the kids on our lap, or in the back. We can help you." I passed the offer (confident of another option that would not be so imposing on the generous driver) and ended up catching a ride from a fellow that was just a few steps behind us on the trail out.
There are two schools of thought, both make sense (to me):
1) Have your backpack on/nearby, stick out a thumb. You'll get a ride from fellow hikers that "feel the pain".
2) Take your pack off. If there are others in your party, separate. Stick out a thumb. The theory here is that you're only seen as asking for "one seat's worth" of space in the vehicle - not you, your companions and all your (smelly) stuff.
I've used both techniques, successfully. With option #2, I do keep my hiking poles with me, at least identifying me as someone that hikes (or owns poles)! I regularly pick up hikers when crossing the Sierra. I can't remember when I didn't get some interesting story or interaction.
Good luck.
