Before You Go
Look around at stores, try on packs with weight in them, ask the people working there what they reccommend. I went with Eagle Creeks' World Journey Womans' Pack. The deatchable day pack was a huge plus, and the back flap zipped over the straps, which was helpful in checking it onto planes and storage in the trains for long trips.
A little will go a LONG way, and depending on where you're going, you'll have chances to buy clothes and toiletries. A pair of jeans is good for any country in western europe- regardless of all the "it's an american thing" hype. Nah. I felt like an outsider at times for _not_ wearing jeans.
Make sure to bring along tampons, birth control, and sometimes toilet paper, if the country you're travelling to isn't a 1st world country. Pack these things in surplus- along with antacids. Lots and Lots of 'em. You can sell them to your friends.
Think about how much film you want to take along and double that amount. Throw in some black and white film, and a few disposables to use while the b&w is in your camera, in case a few colorful sights come to your pleasing.
Travels in Europe
By far the most used form of transportation you'll use is the trains. Each country has their own, usually classified by "D" (fast train), "IC" (Inter-City), "ICE" (Inter-City Express), "IR" (Inter-Regional), "RE" (Regional Express- many stops), "RB" (Regional Bahn(train)- also many stops), along with the "Eurostar", "City Night Line", and "Thalys"- more expensive, Posh trains- often worth the extra price.
Check on plane prices when travelling within Europe. Oftentimes the prices between trains and planes aren't too different, and 20 hours on a train versus 4 hours on a plane will save you a day of travel, a day of recovery, and usually land you a meal- even if it is airline food.
Renting a car with a few friends is a good way to have schedule freedom and allow you to see areas of the countryside not seen via train. Be aware of gas prices however, remember it's per _liter_ and not gallon. Don't bother getting an International license- it's usually not needed.
Info. and Reading Materials
A good guide book will get you started, but don't use it like a bible. Put it down and just get lost once in a while.
Most bookstores can get you detailed maps of the countries you're seeing. If you plan on spending an extended amount of time in any one country, get a map in advance, familiarize yourself with some common roads, and find where the train stations and city centers are.
Even a small book of phrases are handy to have with you. A pocket-size book with food names, common questions (like "Where is the bathroom?"), and some nice comments will get you far and make you feel much less ashamed of knowing only english, when everyone around you is speaking 3 languages at once. And learn 1-10. You won't believe how much you'll need to know amounts.