You got that big intimidating guy look to ya, it ain't easy for us to get rides. I feel like I gotta put a little extra on my smile. Go a little harder on my sign art/humor. Pretend I'm a little more joyful than I really am to be standing in the sun going nowhere without some help. Pack placement is a factor I believe. Bigger guys take up more space in and of themselves.
So if a driver is cruising along in a smaller car and they see my big 6'3" 300 lb ass and then a giant pack and sleeping bag, guitar case, etc.. it just looks like a whole lot of space requirement and I think sometimes just that split second of contemplation the driver goes through can make the difference. They're too far past you to trouble with it anymore, they just keep on going. So tuck the pack behind where you stand. Shadow your carry-ons.
You're probably well seasoned, I doubt I'm teaching you anything new but just figured I'd throw those things out there just in case or for the other large hitchhikers with less experience. I think it's actually worth thinking about even if you're not a big dude. Or if you're a party of two, that's definitely going to have them contemplating how much room they have for you.
I believe they're going to decide more or less by the time they reach you or not much past you. You have such a tiny window of opportunity to win them over, I try to eliminate any additional factors for them to ponder. Good luck brother, sounds like you're gonna need it if you're as close to a prison as Dameon had mentioned.