It's a big country! There are hundreds of amazing races, most of which require travelling. So here's my virgin's racing 101 for the States.
Book flights early, prices fluctuate. www.skyscanner.net is a good resource and you can set alerts for when prices go up or down.
Unless the race is in a really remote area don't book accommodation early. Wait til the day before especially if out off season. I've paid as much for a flea pit as for a 4 star room. www.room77.com is my favorite site for accommodation. Use with www.tripadvisor.com which is pretty good for rating the best and worst!
You definitely get what you pay for in accommodation here. I've worked out that the price in dollars is directly related to the thickness of motel walls! $100 will get you twice as good a night's sleep as $50, seriously. The worst experience you can have on the night before a race is either hearing the TV blasting next door, or worse a 'party' next door. About to run 50k in Sacramento, in a hotel room on your own, eating one minute noodles and 4 people making out next door, doesn't make distance running seem very appealing!
Take road and trail shoes. My limited experience of US trail races (5 to date) has pointed to road shoes unless theres been heavy prior to or predicted during the race. Even the most technical of trail races I've taken part in had a lot of gravel, pathways and manicured single track. Ask the organisers when you check in wat condition the course is in and has there been recent rain.
Look at detail where and how often the aid stations are on the course. Often the course description will give both distance and time (for 1st, middle and last runners) between aid stations. I've found the aid stations to be veritable buffets with water, coke, electrolyte drinks, gels, sweets, bananas, potatoes, sweets, chocolate, energy bars. This has been my experience at every race. Some races are cup free and we were given a handy little plastic cup at the Chuckanut 50k.
A lot of runners will just carry a hand bottle, no pack.
Run naked if you want! It must be an American thing, but there seems to be a propensity to run topless here, regardless of the conditions. Pinning your number to your shorts is considered cool. The skimpy shorts, no top and beard looks is still seen regularly here. Nice for the summer or to show off those pecs and tattoos, but the Americans do it regardless of the weather. Mainly the men though :)
And finally stay around at the finish. Trail runs here, especially the ultras have a great finish area atmosphere with (yet more) food, including all the savory stuff you've been craving for the last 8 hours as you glugged coke and squeezed in gels. Oh, and there's normally beer from a keg and a chill out area to swap war stories.