1) The Back button is your friend. Always and forever.
2) Fanfiction.Net is called the Pit of Voles for a reason. The badfic outnumbers the good. This is not the best place if you hate badfic with a passion and it makes you despise all fanfic. Look for a speciality archive.
3) If you insist on surfing FF.Net for goodfic, try to avoid the big fandoms with sexy actors. For those sorts of things, either look for a specialty archive or let your friends read and reccommend to you. Randomly surfing can be fun, true, but it also leads to screaming at the computer. ("No! Not another Legolas mpreg! The humanity!")
4) Learn what squicks you and avoid stories that warn of having them. (See above note on mpreg.)
5) In fact, learn the jargon. (MS, OFC, OC, OOC, IC, OMC, Sue, Stu, slash, mpreg, fluffy, sappy, angst, wangst, purple prose, urple prose, etc.) It'll help you immensely.
6) If the archive doesn't provide summaries before you click into the fic, be very careful about reading. (See the first note.)
7) Archives that leave reviews are nice. If you're uncertain about a fic, check the reviews. See how they match up with your internal filters. If too many of them give you red flags, think about skipping this story.
8) If you like one of a writer's stories, chances are, you'll like the others. Go ahead and check. Also, she may be able to reccommend other fanfic you'll like.
9) Just because an idea sounds good in theory doesn't mean it will be good in practice. See above note on the Back button.
10) Crossovers rarely work. Be ready to use that Back button if you go into a crossover fic. (If a crossover fic does work, try to leave feedback about that.)
11) If the summary and/or title are misspelled, it's just going to get worse from there. Stay away! [Thanks to Ryuu for this one!]
12) Developing internal filters that let you scan a page of summaries and pick out the ones you should avoid takes time and sporked eyeballs. The more you learn about fanfiction and what people write, the more you learn to avoid stuff you don't like.