Just out of curiosity, do you have auto-stats turned on? Is it not doing a good enough job for you?

You can see the following from BOL to get more information on updating statistics.

http://msdn.microsoft.com/en-us/library/ms187348.aspx

You could use the example to update all the stats for a particular table and wrap that inside of the undocumented sp_MSforeachTable stored procedure to recompute stats for all of your tables...

-Luke.