I'm sure you are using IStockphoto, it's a great site and makes our work so much easier. But be aware that if you buy credit in advance they will be valid only for one year. I knew that and bought 600 credits a year ago for our firm. After a year you cannot be sure anymore when the will expire and IStockphoto doesn't send you any notice. So mark the expiration date in your calender - maybe mark a few weeks earlier so that you won't lose them, like I lost 201 credits.
I've been contacted by Istockphoto, they are watching the blogsit seems, and they reactivated my credits. They postponed the expiration date. Great costumer service - so if you get close to the expiration date and have a lot of credits it might be a good idea to contact their customer service, they may postpone it for you as well.
Update March 2010
The famous expiration emails don't work with the corporate accounts.