I gave up on reusing the plastic bottles. It's so hard to get the old flavor out of the plastic. Possible, except for some flavors, but is more work than I'm willing to do.
What I ended up doing was buying glass bottles
from here. They'll ship small quantities, and the prices are pretty good. I clean them by running the big ones
through the dishwasher, and hand-washing the little ones. I haven't found the need to use a bottle brush... vigorous shaking does the trick.
P.S., don't run the caps through the dishwasher. It'll destroy the cap liner. I just rinse these under hot water and call it good.