Throw bottles away after 6 months???
Yipes. I have bottles that are 6 YEARS old.
I wash them out as soon as I get back with soap and hot water and air dry them completely. Knock wood, but no problems with mold or mildew. Maybe when my LBS offers me a free waterbottle at an event, next time I should take him up on it!
I also picked up a "sponge on a stick" - for lack of a better term - from Bed Bath & Beyond (or maybe it was Linens -n- Things) designed for cleaning down into bottles. Works great for my coffee thermos, too. I got it in the pots and pans section of the store and really gets into the corners.
My camelback bladders I've recently started storing (empty) in the freezer upon the recomendation of some folks, to prevent mildew growth in them.