There used to be several sites with information on how to tweak your dialup settings. Just do a google for that.
I used to play a lot of starcraft over dialup, the only time I had lag issues were in games where someone else was having bad lag too, 90% of the time I could just find another game and it would be ok. Also perhaps you just got a bad conneciton to the server, disconnect and reconnect. If it still doesn't help switch servers.
What speeds are you dialing in at? Anything less than 42,600 is going to suck. I probably wouldn't attempt to play if you are getting anything below 48,000 though.
If you are getting below those speeds then either your dial up modem sucks, or your line is dirty. Call your phone company and complain about noise on the line.