In January, I took one of those small multipurpose zip ties (like plumbers and carpenters use to bundle things together), put it through the key ring of a carbinger clip, then zipped it tight around the indented part of the top of my Brita bottle. Then using the carbinger clip, the bottle was easy to clip on and off to a strap on my backpack or fanny pack (depending on which I was wearing that day). For my trip in two weeks I bought a Baggalini bag that has a mesh pocket on the side for the bottle. It is a very snug fit, but I've placed the Brita bottle in there right after I got it, left it, and am hoping it stretches a bit more. If it doesn't and is too tight, then I've bought an insulated water bottle carrier at Nutztogo.com. The straps will come off. I'll put the water bottle in it and just use the same mutipurpose tie/carbinger clip set up I used in January to clip it to my Baggalini bag, since I don't want to deal with the long straps.
Then again the bottle carrier has a small pocket on the front if it, that's the perfect size for AP, DDE card, CC, and a little cash, so I may leave the straps on it to wear around my neck and then just carry a very small messenger bag that I have for my cell phone and camera. The messenger bag I won on e-bay by the way, from a seller in England and it's really nice. It's very small (about 8"x7"), but has four zippered pockets on it--one big center one, then two on each side. I can put my camera in the center pocket, my cell phone in one of the larger outside pockets and still have three pockets left over.