I'll second (or third, or fourth) what everybody else said.
After you open it up, blow from the mouth end to get as much
juice out as possible. Make sure to do that over a paper towel or the sink because a lot of
juice will usually come out.
Then I use a very small pair of pliers to gently hold the carto while I run hot water (as hot as it gets) from the faucet into the mouth end. I will turn it so I can be sure there is a jet of water coming out of the hole on the battery end.
Blow it out again to get as much water out as possible.
Repeat all of the above until you're satisfied its as clean as you can get it.