I don't want to pretend to be this great traveler of the world. On the contrary, I am still an amateur. However, I'd like to share some recommendations based on my experiences.

Do your research
I ended up having to plan the trip to London for myself and four of my friends. I've planned weekend trips to Orlando and Tampa, but I had never planned anything of this calliber. The first thing I did was research. I checked out travel books from the library, searched random Web sites and took advice from those who traveled out of the country. It's very important to find out what is allowed on your flight. Because of past events, we are no longer allowed to bring any liquid items on board. This includes liquid make-up, eye drops, bottled water, etc. Do your research to find out the complete list of these items. Research can also help you make an itinerary, buy cheap plane tickets, know what part of the city is a safe place to stay and the customs of the country. Some helpful Web sites are Rick Steves and Frommers.
Shop around
Don't assume that the best flight to take is the one you book with your travel agent. I shopped around for a couple of days and found a great site called Student Universe that gave discounts to college students. So, instead of paying $800 for a round-trip ticket I ended up paying half that price. Also shop around for hotels. Based on your research, choose a hotel that is near a lot of sightseeing areas. Most importantly, base this decision on how safe the neighborhood is. European hotel rooms are much smaller than what we are used to in the United States. Many don't have air conditioning either, so if you plan to travel to Europe during the summer ask if they have AC. A small, hot room can make your stay very stuffy. My friends and I stayed in a hostel which was clean, cheap and offered continental breakfast (this is a great way to save some money!). We really enjoyed the atmosphere of our hostel. We ended up meeting a lot of people our age. So, if you opt for a hostel read up on customer satisfaction ratings and comments.
Respect the culture
Before you leave for a foreign country, pick up some of the language. You don't have to be an expert but learn some common phrases like: Those are a few examples, but you get the gist. Along with language, learn the customs of the country. Find out what gestures are prohibited or naughty. In some countries, it is improper to show the bottom of your shoe or the palm of your hand. Wear the proper clothing. When I went to Spain, we had to cover up our shoulders anytime we went into a cathedral. Abide by these customs. Remember, you are visiting their country and must respect their culture.
Once you are there
Finally, you've arrived at your destination! Always bring a map with you. You never know if you're ever going to get lost, but sometimes you find the best things when you are lost. I highly recommend any guided tours around the city. These are really interesting because the tour guides know some cool facts you may not hear anywhere else. However, make time to sightsee on your own. Sometimes these tours go to each place more quickly than you'd like. Keep your money and passport on your person at all times. I recommend a pouch you strap around your waist and keep under your clothes. Robberies do happen. I saw a man forcefully steal a woman's purse right in front of my face in Madrid. Needless to say, I made sure my money was always hidden. Also, don't be afraid to try new things! You'll never get the full experience of your trip if you aren't willing to step outside your comfort level. So, wherever you plan to go I hope you have an amazing time! Happy traveling!!!