Step 1: Background
Naturally, the first thing I discovered was the robot's diagnostic mode... this was before I knew such a mode existed. Here's information on Diagnostic Mode for the older generation. Pay close attention to over current warnings as these will cause damage in the long run.
Below you'll see the classic Roomba spiral footprint on my ugly apartment carpet.
iRobot recommends cleaning every 2-3 cycles (the original manual says 5-10 cycles I think). Die hard Roomba owners clean their brushes every use. Not to worry though, it takes a few minutes - much less time than vacuuming would. Cleaning keeps everything running more efficiently, cooler and thus allows a longer robot life.
Step 2: Begin Cleaning
Now, flip your Roomba on its backside.
Step 3: Remove the Vacuum Service Cover
Remove any dust inside and set the cover aside.
Notice the interesting eddy patterns caused by the spinning motor and dust slowly clogging from one side.
Step 4: Remove Brush Grille
Now remove the top middle section and then press in on the sides to remove the left and right section.
The grille should now be out. Clean as necessary and set aside.