A lot of packing a suitcase depends on where you're going.
If you're traveling with your family on spring break, leave room for souvenirs. You'll end up raiding the hotel gift shop on the last night desperate for trinkets for your friends back home, and you won't know how much space you'll need. If you're going on a trip with your school, don't bother with the drugs or condoms. Let's be honest - you don't have any friends to do drugs with, and unless you plan on wrapping a hot dog with those condoms for an unsuspecting student to find, you're not gonna need those either.
Find out your boyfriend cheated on you? It's tempting to throw his clothes out the window sans suitcase, but don't take the easy way out. Jam his personal effects into that nice rolling Rick Steves bag he bought for his business trip, toss in your cat's most recent batch of kitty litter, and wait for your cheating-whore-of-a-man to return from work. As soon as you spot him outside, bombs away. Aim for his windshield.
Breakups happen, and it's okay to get upset. When your girlfriend of four years tells you she's just not sure which direction the relationship is going in and that she doesn't really see a future with you and you'll just be better off without her and take all the time you need moving out, she means you better be out of there when she gets back from jazzercizing. Turn on some sad music and spend your last few hours packing away all of your memories: the shirt you bought at the first concert you went to together, the picture of you two on Steel Force, your grandmother's ring that you were planning on giving her tomorrow night. Just remember, big guy - it's okay to cry.
Going on a music trip with your marching band? Don't bother packing at all! Instead, spend your time avoiding the inevitable mountain of homework due when you get back making fun of the PSSA's you took two weeks ago and hoping people remember what you're talking about.