I agree that getting from Whitney Portal is a very easy hitch. All those hikers and backpackers can relate to another hiker and want to help out. If you want it to go even faster,
- Have a sign. That could be packaging from a trash can or from the store at Whitney Portal and drawn on with a Sharpie. Those FedEx Tyvek envelopes that double as stuff sacks, rain hats and soup bowls? Turn it inside out and use the white inside as a hitching sign. You can do that at home. Make the letters BIG. Surprisingly BIG, not for Whitney Portal, but for any highway use.
- the sign should say something really close (Whitney Portal in this case). Even if you are going further. People hesitate to pick up someone for a 300-mile ride. Once they've picked you up and found you're not weird, awkward, or an ax-murderer, you say, "Anywhere in Whitney Portal is great. I'm trying to get to LA."
-stand in a place where people can see you in advance, are driving slowly, and can safely pull over to pick you up.
- make eye contact
- no sunglasses
- hang out the most interesting flag you're remotely entitled to associate with. I'd use an Alaskan flag, for instance. This is VERY handy away from a hiking area and for foreign travelers. It makes you more interesting. It explains why you're by the side of the road without a car (i.e. it distinguishes you from local poor people). It provides a starting point for a conversation. For Americans in other countries, someone might want to practice their English or complain about US foreign policy. You get a ride either way.
- From fastest to slowest at getting rides: single women, a pair of women, boy-girl couple, single guy. Multiple guys should just get a bus ticket or plan to walk.
Edited for typos.