Are you cleaning them before they're actually empty? You should, I think, since I'd imagine that would allow
juice to 'bake onto' the heating element.
I've only cleaned 2 CE4s so far, but here's what I did, for what it's worth:
(I put everything in an old medicine bottle, so I can cap it and trash it responsibly so it doesn't get in the water supply. I'm a hippy like that! :wink

Have a cup of hot, clean, preferably distilled water handy. (Hard water may leave a buildup of lime behind)
1. Take off the black mouthpiece. Rinse it with hot water and shake it dry. Set aside.
2. Dump out any remaining juice into the med bottle.
3. I use a syringe to squirt down the insides of the ce4 with the hot water, til it's half-full. If I didn't have a syringe, I'd use an old eye dropper.
4. I plug both ends with a finger and thumb, and shake it for 15 seconds, then dump that water into the med bottle.
5. I repeat steps 3 and 4 four more times, or until I can't smell the juice any more (or too much, at least.)
6. After the last water-dump, I hold the bottom of the ce4 (the part that screws onto the battery) and flick it the way you shake down an old-fashioned thermometer before using it, in order to get as much water out as possible. Don't let go!
7. Screw the empty ce4 onto the battery. Don't put the mouthpiece on.
8. Dry burn until vapor comes out for a few seconds, then let go of the button and wait several (if you're me, that means 10) seconds so that the heating element can cool down again. (No one told me to do this, but I'm just paranoid that reheating it too quickly while it's dry will fry the heating element.)
9. Repeat step 8 until very little vapor comes out. (if you're me, that means 4-5 times)
I admit, I'm of the 'under-do it' school of thought. Getting most of the juice is out and tolerating the first few transitional hits is better (in my book) than being too thorough and frying the cartridge.
Please, Jackstraw, don't try my technique until / unless anyone with more experience than I, says if I'm doing it right first!
*The second sentence should have read "You should
NOT 'smoke them dry', I think, since I'd imagine that would allow juice to 'bake onto' the heating element." Sorry!