Clothes tend to make up the bulk of most travelers' suitcases, and reducing the number of outfits you pack can lighten your load significantly. No one wants to run out of clean underwear in the middle of the Brazilian rain forest, but it's possible to find a comfortable balance between wearing the same stinky jeans and T-shirt the whole trip and changing your outfit three times each day like a celebrity.
Top Tips:
If you're going on, say, a seven-day trip, spend a week before you leave keeping track of everything you wear. Make a list, or, if you learn better with visual aids, keep these items together in a laundry basket. Then figure out which items you can do without.
Bring clothes in neutral colors that you can mix and match, and only pack shoes that can be worn with multiple outfits.
Check the weather at your destination before you leave, and pack accordingly. If the weather deviates significantly from the forecast, you can always buy a sweater or rain poncho and keep it as a souvenir.
Many travel supply companies sell small packets of laundry detergent (you can also find these at a laundromat). It only takes a few minutes to wash your clothes in your hotel sink and hang them on a hanger to dry. When you wake up the next morning ... hello, freshly washed clothes!