Here's my advice with samples:
-Generic (clarinets.us, customersupport.us)
-Local type industries that may not want to appeal to the whole world with their website (plumbers, bakers, dry cleaners, etc. - exterminating.us, truckdrivers.us, bikerepair.us)
-Acronyms or numbers (three letter or number, easily usable- syo.us, cpr.us, 680.us)
-Single word, strong two word (riverboatgambling.us), extremely strong 3 word (creditcardhelp.us) or common phrases (theunitedstatesflag.us).
-Geographically related to US (cities, places, tourism, etc - mammothcave.us, manhattannewyork.us, butlercounty.us)