Buy a money belt, either one that goes around your waist or your neck. Wear your money belt at all times. Keep your money, credit cards and passport in the money belt. Do not keep anything in a purse or pocket that you are unwilling to lose.

Carry an umbrella or rain poncho if you are going to a rainy destination. If you are going to a particularly rainy locale, consider a protective covering for your pack as well.

Try to avoid lugging around an entire guidebook. Chop it up only into the regions you are vising. Alternatively, print off information, discarding as you go along. I recommend against having all guide information on an electrical device -- you could end up stranded if the battery dies.

Bring a few self-contained snacks such as granola bars or packages of nuts. Alternatively, buy in-country options. You will be grateful later when it appears every store is closed because it is Sunday or some holiday you didn't anticipate.

Bring ear plugs. You will sleep better. A neck pillow is useful for the plane and hostels with thin pillows. Buy the inflatable kind.

Bring tissue, either Kleenex or roll-less toilet paper.

To sleep-sack or not to sleep-sack -- I have carried sleep-sacks all over Europe and barely needed them. That said, I have also done without and had to pay a bedding fee. Plan ahead -- most hostel websites will tell you if there is an extra charge for bedding.