• If you've got a Capital One credit card, use it on your trip! Capital One is one of the few credit cards that doesn't charge a foreign fee when you use it (most cards charge around 1-3% per transaction which can really add up).
• Some banks offer no fees at certain ATMs. For example, if you have a Bank of America debit card, you can use it at any Barclays ATM in the UK or Deutsche Bank in Germany without charging you extra fees. Most experts claim that in the end, it's cheaper to withdraw money from no-fee ATMs and avoid using the credit card as much as possible. However, make sure to call up your bank or credit card companies and have them put a note on your account that you'll be abroad for a length of time so they can override any blocks or holds that may occur.
• If you're visiting a place that has excellent public transportation, consider getting a travel card rather than paying the fare each time. It's much cheaper (and an excellent way to do as the locals do). London has the Oyster card that you can reload for its Underground; Tokyo has the Suica card that is interchangeable with other railways.
• Study up on general hospitality and common phrases. Not only is it important to know how to ask for directions or order a drink, but it's also worth knowing the tipping customs. As one of my co-workers mused, "I tip everywhere. The world is my strip club! But what do I know? I've never been abroad!" However, that mentality is considered quite "American"...during a trip to Japan, a friend of mine told me how tipping is seen to be demeaning to the person being tipped.
Got your own tips? Share it with us in the comments!