Getting an analog modem above 56k on a phone line is impossible. Even if the hardware on your ISP's end was capable of greater than 56k by some freak of nature, the voice portion of a telephone line eventually gets jammed into 64k of bandwidth anyway. Yours should be able to train above 50.6 though, if your phone line, modem, and isp's hardware is perfect (I think 57600 is the fastest I've seen reported by windows.)
As for other ways of getting dialup to go faster, 56k V. 90 was the fastest analog protocol that was implemented in any decent manner, so there are no more firmware upgrades for you to get. The way download accelerators work is to compress data like text, crappy images, etc, before sending it across the line. They won't help you download mp3s and things faster, but it'd probably help with checking your email, etc. As far as I can tell, all the registry mods and such things available on the internet are not worth looking at.
One amusing thing that could be done, if you had two phone lines to waste, would be to get two modems and multiplex them, the same way that multiple internet connections are load balanced for small servers and such. I have never seen anyone with this setup on dialup, and think it'd be hilarious.
P.S. The reason that analog modems never went faster than 56k was because they would have had to break transmission regulations in order to do so.