This is how I clean mine, and I'm still using cartos that I've had for almost 7 months, now.
Cleaning Method
1- I first remove the little white caps, and place them into a empty water bottle, with some dish detergent & very hot water. I shake those several times, while I'm cleaning my cartos.
2- I throw all of my cartos into a pot, and fill it with hot water and boil them for 15 minutes.
3- After the 15 minutes is over with, I rinse the cartos, in a colander. Then do another 15 minute boil.
4- While the cartos are boiling, again, I rinse my little white caps, and put them back into the empty water bottle, with a little more dish detergent, and very hot water and shake them again.
5- Then I rinse everything, in the colander, after the second boil.
6- I grab some towels, and proceed to blow all of the water out from the top end of each carto. (Gives the lungs a good workout, in this process! LOL)
7- Then I stand each one up, on the top ends, and let them dry out for 48 hours. I haven't had any problems, with the exception of maybe 2 or 3.