I take off the tip and then fill it with hot water, put my finger over the top of it and shake it like a mad woman. Repeat 8-10 times. Then I fill it with hot PGA, and let it sit filled all night long. The next morning, I dump out the PGA, sling it hard a few times to get out as much fluid as possible, and then put it up somewhere for, a few days, (tip off), so that it can get completely dry inside.
After that, they work just fine as long as I give them time to properly wick the new juice I put in there. I have at least a half-dozen of these that I have been using, cleaning, and re-using at least a 10-12 times each with no problems. If I get any minor cracks in them, I seal those up with clear nail polish. If the top cracked off of it, I would just chuck it, tho.
My only issue with them, other than the fact that some juices cause the cracks, is the fact that they mute flavors by at least 25%.
~Tiger