Forgetting to call your bank before abroad is a common error that even frequent international travelers make; it slips your mind until the plane touches down, and by then it's often too late. These days banks have almost zero lag time in noticing a debit or credit card being used abroad, so you will get shut down on pretty much your first swipe in a foreign country.
While traveling internationally, each time you go get cash you will likely incur a fee of some kind. These can vary a lot depending on whether the ATM is run by a large bank or not, if the bank is on your card's network and more. Keep in mind that fees can change from year to year, so it's worth checking before every trip.
DON'T FAIL TO MAKE A PLAN FOR GETTING TO YOUR HOTEL FROM THE AIRPORT.
That first taxi ride from the airport may be your most financially vulnerable moment of any trip -- the time when you have no idea how far it is, what a fair price is for the ride, whether you have lower-cost alternatives such as a train or bus, or even whether your hotel has a free shuttle. It is best to figure all of this out before your trip; when you arrive you are tired, often without much cash and carrying a ton of luggage. You don't want to be fumbling around trying to figure all this out on the airport curb.
Many travelers will leave it until the end of a trip to tip the housekeepers (usually by leaving some cash on top of the bed); this can backfire. On any given stay, you might have a different housekeeper from one day to the next, and to be the most fair and to get the best treatment, it is best to leave something each day. Many housekeepers live on subsistence wages, and this is both the right thing to do and also arguably a bit of blood money.
DON'T LEAVE YOUR VALUABLES UNSECURED.
Shoving your wallet in your back pocket or your purse over your shoulder is such a natural and almost automatic action that almost all of us will do this at some point in our travels. It is also the easiest way to get pickpocketed or ripped off, as these spots are both obvious to thieves and difficult to protect.
A button on your back pocket or an across-the-body bag instead of an over-the-shoulder purse can also help here; adding just a little bit of difficulty is often enough to inspire thieves to look for other victims.