I have hitched rides in Sequoia, Kings Canyon, and Yosemite.
It helps if you look halfway clean, and I know how difficult that can be. It helps if you have a small pack that will be easy to throw into a vehicle.
It helps a lot if you have a small sign (RIDE?). If you want, use a black marker on the blank back of a topo map. If somebody does stop and roll down a window, you can tell them exactly where you are trying to go.
If helps if you are standing upright by the edge of a parking lot which gives the driver plenty of warning and space to stop. If you are collapsed on the ground, they think you may be a problem, so they keep driving.
Hitching a ride is kind of unpredictable. I've waited for hours and hours on a hot day before, and that was no fun. I've also gotten a ride within the hour. Prepare for the worse.