Whatcha sending? Executables? Pictures? Zip is a pretty good compression, other programs might do better, but they will only improve by percentages.
I feel for you on dial-up, that's what we're using. Pictures take forever, and they don't compress worth beans. Are you using a dial-up accelerator? It's no magic bullet, but it does make using the dial-up tollerable.
The best thing to compress is text - that gets the greatest compression. Executables do okay, but not as good as text. Seems to me that MP3's and JPEGs have the worst compression, and it's generally not practical to compress them. If you only need the files to be at work when you arrive, what about CD or DVD?
Regards,
Pat