having hitched 1000+ miles as a 22 year old chick: get their license plate, text it to a friend, including vehicle and driver description. i usually don't do this... but maybe i should haha.
***even better advice: once in the car, smile, give a "hey! you just made my day!" and, making sure the driver hears you, phone a friend: "hey sarah, just got a ride from a kind couple/trucker/family, i'll be in town in 4 hours, see you then!" regardless of whether that friend will be in said town. you wanna make sure the driver is aware that someone knows your whereabouts and is expecting you.
also, you could ask what route they're taking; be familiar with routes you should be taking, so you'll know when you're heading to sketch backroads [[my girl-friend once had a dude drive her past a requested gas station onto backroads only to ask, 'well, uh, can i at least see your tits before you get out?' she vehemently said, 'no, get me the fuck back to the station.' which, luckily, he did.]]
weapons... aren't so effective if you're cruising at 70 mph, but perhaps if the driver pulls over! i carry a knife always for my peanut butter and apples! hopefully never for someone's jugular!
that said... people are actually super awesome!!!