Apply for passport. If you have one, check the expiry date.
Arrange for any visas that might be required.
Order your railpass.
Start reading your guidebook. Highlight anything that sounds interesting or that you want to remember.
Get copies of any prescriptions from your doctor. This is especially important if you are diabetic or have any other condition that requires you to carry needles on you. Bring extra medication with you and keep it in the original bottles.
Get a copy of the prescription for your glasses or contact lenses.
Put together a small first aid kit - pack the following in a heavy duty ziplock - bandaids, antacid, antidiarrheal medication, antihistamine (allergy) pills, Ibuprofen or something similar, antibiotic ointment, cough and or flu medication, tweezers, antiseptic wipes. These things are OK out of the box as long as it says on the back what it is.
Check the expiry date on your credit card to make sure it will last the length of your trip.
If your ATM pin isn't 4 numbers, arrange to have it changed.
Purchase a small amount of travelers cheques for emergencies.
Make copies of all important documents. Leave one set with someone at home and take one with you. You can also scan the documents and send it to the email address you are going to use while travelling.
Purchase travel insurance.
Break in any new shoes you are bringing.
Set up an email address you can access from Europe.
Make a note of your favorite websites, like say guideforeurope.com perhaps, or those you might need, in the back of your journal or send yourself an email listing them.
Put on your backpack and go for a walk. Adjust it so it feels comfortable. Wear the hipbelt and don't allow for space between your pack and your back - it shouldn't be pulling away from you.
Do a trial packing run to see if what you are planning will fit. Don't fill your pack completely, leave room so you can bring home a few things. Roll your clothes to cut down on wrinkling, store things that could spill in ziplocks.
Buy small locks for every zipper on your pack and daypack.
Book (at least) your first hostel. You don't want to show up with jet lag and try and find a bed - it won't be pretty.
Confirm your flight 2 days prior to traveling.