I gave up on these because no matter what I did they got grossly moldy all the time. I did the q-tip thing for a while, but then I finally switched my son to a Sigg with a kids top.

Two kids and never used a sippy cup with a straw or valve. The plain cups from Tupperware that can also be used without their lids worked just fine. Yes, they can spill, but my kids only drink in the kitchen so it's no biggie. I also know that at our daycare they hate cleaning these cups so much they've banned them.

at the beginning (with the infant) we used Medela Quick Clean Micro-Steam Bags: dump all the small parts of sippy cups, baby bottles, dummies, etc. add some water, zip the bag and microwave this for a few minutes.
Later, we used sippy cups ONLY for water and ONLY at the bedtime, they never got really dirty. But we were picky about what kind to buy (how easy it's to clean), and used Q-tips.

Moldy? I've never had it happen...I just run hot water through the one we are done with, then throw it in the Munchkin dishwasher holder thingy. The dishwasher uses hot water and steam and they always come out clean. There have been times when we've left a milk sippy out overnight and ew...milk gunk! Gross. Luckily, we use the plain Rubbermaid re-useable drinkboxes. The replacement straws are cheap. I just toss it and use another. I know, throwing them out every now and then might not be totally eco-friendly...but its a time-saving, affordable alternative for us. I'm thinking my 23 month old is ready for the sport-style sipper now. Before now he would just gnaw them and having to tip them up ALL the way so he could get some, with his neck at a contortionist's angle seemed too weird to me. I think he has finally figured out the right way to do it :)

Soak in a bowl of 50/50 white vinegar and water. Rinse.

The Sigg cleaning brush set is great for cleaning straws and small parts.

I use the same method as complainypants and have never had a problem. Rinse them with hot water pretty quickly to get the milk out of them (or juice) and then put them in the dishwasher's top rack. I've never had any mold or any other problems and they get clean that way.

we keep a plastic bin in half of our sink that is ONLY for cleaning sippies and parts. (with triplets, we go through a LOT and i don't have time for really fastidious cleaning.) once a day, we soak them in v hot soapy water and rinse them off. we always rinse them out right after use. we use the playtex sippies, so the valves are pretty simple and don't get clogged.

every once in a while, we either clean them with the medela steam bags or run them through the dishwasher.

we only use kleen kanteen sippy cups. they fit avent spouts. We completely disassemble and run them through the dishwasher daily. We've never had a problem.

In the past, we drank from tupperware drink containers with straw spouts at work. THOSE got moldy regularly (YUK!) we had to soak them in 25% bleach for hour at least once a month... even then, there were issues. I think a good portion of the problem stemmed from the fact that they couldn't fit on the top shelf of the dishwasher, and instead had to be handwashed.

we switched to kleen kanteens and we've been fine ever since :-)