If all else fails, you can fall back on style appropriate-ness. Are you making a continental lager? Try noble hops - Saaz for milder, Tettnang or Hallertau are a bit spicier. An English beer? East Kent Goldings are always, always delicious, but if they're too assertive Fuggles are milder and grassier. American ale? It seems like every micro and its brother are using Cascade like it's going out of style. If you're making an American take on a foreign beer, you can use the American spin on that style of hops: Willamette for Fuggles, Sterling for Saaz, Mt. Hood for Hallertau, go nuts. American IPA? Try high-AA hybrids like Chinook, Centennial, Simcoe, etc.
This is a vast oversimplification, mind you. Mainly cribbed from different places I've read. As you use them, you'll get to recognise their flavours. Try sniffing each variety before you throw it in, or put a tiny piece on your tongue and then quickly remove it.