haha. that's an interesting approach.
i am experienced in cross-country and local hitchhiking. i've never once had someone request sex. and i'm a hot chick. (http://www.photobucket.com/albums/y134/dystopiate
) i just dress like myself. hippies and mexicans are usually the ones that pick me up. or really stoned people who think it'd be an adventure. i've meet a lot of good friends and connections that way. if you look like a hippy, hippies will want to pick you up. they won't want anything in return.
i do see it being probable that someone would want sex or money in return. just tell them no, and if they won't drive you without it, get out of the car whereever you are and wait for the next ride. i guess rape, kidnapping, and murder are also possibilities. personally, i find this risk exilirating.
a clean-cut look could get you more rides in certain areas, fewer in others. with conservative drivers, you might want to tell some "damsel in distress" type story. say that you wouldn't be hitchhiking if you weren't in such dire circumstances. make up some story about how you've got to get to such-and-such, and your car broke down, and you're from some other state and have no family or friends to rescue you. they might even through you some cash with that one. but you have to be prepared to have good answers to follow-up questions on your story.
with anyone that looks like they might not freak out too much upon hearing it, just tell them the truth that you're hitchhiking for the hell of it, to get across the country, to see it along the way.
and you might want to familiarize yourself with the US highway system. figure out what would be the best roads to take to get to your destination, or to the next road leading there, or to a truckstop or something where you might find another ride. because most people will only take you so far.
why do you want to go to portland? are you willing to sidetrack or abandon that goal altogehter? that happens sometimes when hitchiking, and that's the beauty in this mode of transportation. for example, i was one trying to get to philadelphia, and i got picked up by these hippies who proceeded to get me stoned and then offered me a job selling magazines door-to-door, which i took, which got me a free greyhound ticket to new jersey. nj is right next to philly, so even if i didn't like the job, i could have gone back to my original destination. but i ended up staying with the job, which was a travelling job and landed me in long island and virginia before i eventually got picked up on a drug warrant.
i've also been hitchhiking locally, trying to get to the train station so i could go to the city (from the suburbs where i reside), but i got picked up by some cool people and just decided to hang out with them all day instead and then had them drop me off back at home.
from what i've experienced, the hysteria surrounding hitchhiking is overrated. you COULD get raped, mugged, malled, killed, etc., but you won't.