Don't put yourself in situations where you are isolated alone with strangers--like going into homes or to parties with people that you are unfamiliar with. If you are on your own, then stay in public areas, and if you are staying in hostels or BnB's, then tell your host where you will be going that day. Always try to maintain your bearings and have an approximate idea of your location, and how to get back to where you are staying. Be careful with how you protect your bag, money, passport, credit card, etc. Unfortunately the areas of Europe that see a lot of travelers and tourists also have a lot of pickpockets and thieves lurking about, so you will need to be on guard.