I divide the amount by 52 and set that amount aside each week in my checking account. I "deduct" it in MS Money, by putting "WDW" in the check number column. Then all I have to do is a search on "WDW" and I delete all the entries and purchase travelers checks with the money.
Luckily I've only had to do this for a few vacations (this one included), but it works well for me. Of course, I do the same thing when I use my American Express card, by placing "amex" in the check number column, so the money is already set aside when I pay my bill, so I'm used to doing this.