Are you using meth?

How often do you brush them?  How often do you visit a dentist for a
professional cleaning?

Once you "scrape them clean" a regular tooth brush (used properly) should
keep them smooth if you use it a couple times each day, especially soon
after you eat.  The key though is to use it properly.  Right now you may
simply be holding the tooth brush in such a way that it doesn't contact the
teeth effectively.  Sometimes, if you have strong muscles in your cheeks, it
is hard to get the brush way in the back, but up front where you can simply
move your lips apart you wont have that problem.   That's probably why your
four front teeth are cleaner--they are easier to reach.

Even the electric tooth brush wont do anything if you don't use it long
enough and don't hold it correctly.   When using the electric tooth brush,
don't just move it over the teeth quickly; make sure you hold it on each
tooth for a couple seconds, and do this for each surface of the tooth
(tongue side, cheek side, etc.).  It should take 2-3 minutes to brush all
surfaces of all your teeth.

You don't want to bear down hard on the brush, just make sure it is making
good contact with the teeth.  If you press too hard, it will do more bad
than good.  Some Bruan electric brushes will tell you if you are pressing
too hard.  Check the instruction manual, and if yours has that feature, use
it to get a feeling of what too hard is and make sure to stay under that
limit.

Also, make sure you have a professional cleaning by your dentist regularly.
While you are there, you may want to brush your teeth while your dentist or
hygienist watches you so that they can point out anything that you might be
overlooking.  They can then show you the best methods to use (it might be
better to see it done than to read about how to do it).

--Stephen D