I lived in Breckenridge, CO last winter and worked for Blue River Sports. I actually did some of this hitch so here is some information to help you out.
I-70 was pretty easy to get rides, just have people drop you off near the roundabouts on the exits and fly your signs there. Make sure you ask people where they are going and not just where you are headed. We had some asshole pick us up last year and drive 3 miles in an area where it was impossible to hitch even after we told him we were trying to get to California. Normally I don't give a shit, but standing outside when its like 10 degrees out sucks.
Grand Junction sucks balls to hitch out of, but you'll eventually get a ride...as for Fruita, don't bother stopping in this area. It's full of rich assholes who won't pick you up. We sat on the on-ramp for two days to get a lift from someone. Despite there being a truck stop, no truckers stopped at all.
Utah isn't too bad to hitch through though we did get hassled by the cops in St. George. They took our licenses and checked for warrants and threatened to give us citations if we didn't leave.
Never hitched in Vegas so I have no advice there. We flew signs mostly that said the state we were going to or the city. GJ for Grand Junction, Utah, etc. nothing too special. Threw a thumb out there too. Make sure especially when going through Utah you get dropped off in towns with people in them. Don't get stuck somewhere in a small town because you might get stuck there for a long ass time.
P.S. I'm gonna go out on a limb and say the best thing you can do to get rides is buy a flag of your country Costa Rica and hold it up or put it somewhere visible that ppl driving can clearly see it. If people know you are from a different country they will probably be more willing to pick you up. Good luck dude.
