Do: Pack less food. I'm a big eater, so I was worried about getting by on fewer calories than usual. My typical menu included 2 cups of mashed potatoes and a hot cocoa for breakfast, 1 package of Ramen noodles for lunch, 1 cup of nuts and 10 dried apricots for snacks, and 1 soup, 1 box of couscous, 4 ounces of smoked salmon, and 1 cocoa for dinner. That's a fair bit less than I usually carry, especially in the lunch and snack departments. But because I was expending much less energy carrying a load of 20 pounds instead of 50 or 60 pounds, my stomach never growled and my muscles never felt fatigued from lack of fuel.
Do: Safeguard your gear against rain. Since you're carrying fewer clothes, it's more important they stay dry. My worst night of sleep ever came after the third day, when every last stitch of clothing got soaked. With no dry spares to change into, I was forced to sleep-dry soggy clothes.
Do: Watch where you step. With a light load, you're less likely to suffer a severe ankle sprain, but any injury could pose a serious problem when you're carrying less food, fuel, clothing, and survival gear.
Do: Be ruthless. If you aren't certain you'll use a piece of gear or clothing every day, don't pack it. Exceptions: 1st aid kit and toilet paper.
Do: Pack a hat and gloves, but leave the extra fleece at home. If the temperature drops sharply, you can always cook dinner and breakfast from the warmth of your sleeping bag.
Do: Consider taking a rest day if the weather gets fierce. Better to catch up on your sleep under the shelter of your tarp than get your gear wet moving out into the storm.
Do: Wash your socks and underwear mid-trip, or carry a spare set. Unless you're hiking solo, that is, and don't mind the stink.
Don't: Saw your toothbrush in half. I cut mine too short and couldn't reach my back molars.
Don't: Skimp on raingear or shelter if consecutive rainy days are common where you're hiking. As an ultralight backpacker, you're most vulnerable to extended stretches of cold or wet weather.
Don't: Depend on a single lighter or Piezo (stove) ignition. Unless you know how to make fire without matches, always bring a backup lighter or waterproof matches.
Don't: Get so fanatical about food weight that you pack boring meals. A few special treats may add some ounces to your load, but I found that a tin of smoked mussels or a squirt of Tabasco in my Ramen noodles went a long way toward relieving bland-food boredom.
Don't: Assume you'll immediately double your daily trail mileage. Walking for 8-10 hours still takes a toll on the body, especially feet clad in light shoes.
Don't: Forget your chocolate! I did, and I was jonesing for a Snickers by day 3.
Don't: Ridicule friends carrying big loads. They might leave you the scraps from their gourmet dinner.