These tips from Productive Strategies for planning international travel are outstanding, and in the post-9/11 world many are relevent for US travel as well. Such as this one:
Make sure you call your credit card company and let them know you plan to be out of the country. Otherwise they may shut down your card thinking it has been stolen. Also be aware that some stores process cards differently, so it is possible that your card might be rejected. Make sure you have other means of payment available.
I used to think that my credit card was good anywhere. But two years in a row I had a credit card frozen on the first day of the MacWorld trade show in San Francisco — with no attempt in either instance to notify me by phone or by email. (I found out when the credit card was refused for a subsequent purchase — inconvenient and embarrassing.)
When I caught up with the credit card company, they were unapologetic. A $49 piece of astronomy software from a Danish company? Clearly my card had been stolen and taken to Denmark. A camera purchased outside of Seattle? Suspicious.
In these days of frequent business travel, I was shocked to discover that buying something on a trip, other than food and a hotel room, can trigger a freeze on your card. While I had previously left most of my cards at home (to minimize damage from theft) I now take at least two on the road to protect myself from the credit card company. And, as the Productive Strategies folks suggest, I call the credit card company nannies in advance to let them know I will be going shopping.