I take my DE filter apart every fall for a thorough cleaning. It's obvious when opened up after backwashing that backwashing isn't sufficient to really clean it.

I thoroughly hose off the grids then use this to clean them:

http://www.amazon.com/BioGuard-Kleen...+spray+cleaner

I've been doing this since 2004 when our pool (and DE Filter) was installed with excellent results.