<hangs head in shame> I had the experience not long ago of discovering some forgotten text ads that were still inviting people to check out the hot new widgets for 2009 ... even though the 2010 widgets had been out for two or three months already. </shame>
Keep an eye out for time-related issues like that. Other than that, though, a good ad won't really go stale unless the same people are seeing it over and over too often.
It's wise to be testing new creative fairly regularly, and simply having new ads in your rotation will help to keep things fresh. However, don't delete old ads until you're confident that the new ones are performing better.
Differentiate yourself, make your ads stand out, eyeballs will naturally draw to the one that stands out the most.