A few general packing tips:

First, to echo what others have already said - don't check luggage unless you absolutely have to. I studied in Europe for 2 months in college and took only carry on luggage!

Pack an empty lightweight backpack or totebag in the bottom of your luggage. This gives you some leeway on space. If you need to, you can always check a bag going home, since if your luggage is delayed is usually easier to deal with at home then on vacation.

Pack a swiss army knife, some wet wipes or Purex-type antibacterial stuff, a travel-size tissue, a few Shout wipes, and some safety pins in an easily accessible place. I usually pack them in my small travel purse.

Pack a few ziplock bags of various sizes. Great for anything that could leak, or packing wet socks, or saving half your apple for later.

Personally, I swear by my travel iron. It weighs less than a pound, packs flat, has a built in voltage adaptor, and is great for getting out wrinkles. It's also great for drying clothes that are still damp in the morning. You can't add water to it, but it works great for getting out packing wrinkles. You can find it at WalMart and Target in their luggage section.

Women's tips:

I love my pair of black jersey matte elastic waist pants. They are the most versitile pants I own. I can dress them up with a blazer or twinset, go business casual with a white blouse and more casual black shoes, I can even throw on tennis shoes and a zip-up sweatshirt for a relaxing wear around the hotel outfit. They're elastic waist, so you can't beat them for comfort, and they go with anything. I got my pants in Nice, France on a trip in 1998, but you can find similar pants at Ann Taylor, Old Navy or any major department store. I like the fitted but not skin-tight look.

I also love my black long sleeved cardigan. It's cotton, slightly fitted and ends just above the hip. Another great dress it up, dress it down, flexible item.

Makeup: pack the necessities. I take concealer, foundation, blush, one eyeshadow compact which has two shades, mascara, eyeliner and two lipsticks. That's it. (Some women take less than this, but this is what I usually wear, and it doesn't take up much space.)

Take the minimum number of shoes you can get away with. On a 10 day trip to Italy in March I took one pair of tennis shoes (many people don't take tennis shoes but in my opinion, you can't beat them for comfort) and a pair of comfortable black casual shoes.

A travel folding hairdryer with a built in voltage switch - Vidal Sasson makes a great one that's available at Target for $14.99.