To ensure the greatest possible flexibility on the bottom, I went looking for a new cast-on. I discovered that Jeny - she of super-stretchy bind-off fame - had found a very stretchy cast-on. However, I found her method a bit awkward and used the method described in this video instead. It's definitely worth a try if you have a large number of stitches to cast on (no long tail) and need great elasticity. It requires a little more attention than the standard long-tail cast on, but is well worth the effort.