buckworks - 4:55 am on Oct 27, 2010 (gmt 0)
Find out what the tax implications would be if you sold for a lump sum versus keeping the site and earning income over time.
Be sure you're making your sell-vs-hold comparisons on the basis of after-tax dollars, not just the gross amounts.
Something else to consider is how your other sites might be affected if you sell a strong site. If you've been doing much cross-linking between your sites, your other sites might see a dip in their traffic and earnings once the new owners start taking out your cross-links and putting in their own.
If you've been doing a lot of cross-linking, it might be wise to assume that your other sites will sag for a while, rather than just staying level.