A rolling suitcase or small rolling backpack is much better to travel with, a regular backpack will really hurt after a while.

Always have clothes to use for layers, you never know what the weather is going to be like or where you will travel to next. Also, the hostel in London does not have any hangers for clothes, so bring your own.

Carry lots of extra batteries if you use a digital camera that requires them! Have an international adapters (both euro and UK)  if you have a laptop computer or need to plug in something.

Pack a lot less than you need, or pack things that you can easily leave behind because you will accumulate a lot of souveniors over 3 months. When you pack initially to go over, purposely pack things you will use up so you have room for things you buy (such as shampoo, snacks, water)

They dont have as good of peanut butter and don't have ranch dressing at all!! Bring your own! 

When traveling to the continent, worry more about clean underwear and socks, other clothes are easily reusable, and make sure to have something somewhat dressy, leave plenty of room in your suitcase because there is a weight limit as to what you can bring back. 

This may sound obvious, but take pictures of everything, friends gathering together and scenery. I only really took pictures of scenery but my memories I have are of the occasions when I hung out with friends. They have computers you can upload the photos to and you can burn your own cds, so don't worry about running out of memory. 

When traveling, always have a bottle of water, you will get dehydrated on a plane and walking around. Its good to carry some food like nuts or bread to eat because you do not know when you will be able to sit down to eat or you might be forced to pay a lot of money for something not that great.

Plan your trips one to two weeks in advance, prices will really soar for Easyjet when it gets any closer to departure date.
Hostelworld.com is a great website for finding cheap hostels to stay at in Europe. If you do stay at a hostel and have to be in a room with strangers, its not a bad idea to take your wallet and passport and put it under your pillow when you sleep for extra safety.

In my experience, people are very nice and trustworthy, but you get the occasional crook you have to deal with. Just keep a watchful eye when on trains or subways, try to stay sober because thats when thefts happen.