Your best bet (contrary to commercial products' claims) is to reduce your connect speed to one that is relatively error-free. Depending on the modem that you have, your command will probably be:
AT +MS
or
AT &N
You can set various rates, but you may want to run it down to 33.6K to start with, just to see if you end up with a performance gain (although 3.4kbs is probably the best that you can expect with a 33.6K connection). You can step it up toward 56K until you start getting a number of errors.
Generally a slow error-free connection will perform better than a faster error-prone connection because of the retransmits that must be performed in order to compensate for the errors. Even if you reduce the MTU size (which is what the connection performance improvement software does).
Good luck!
pansophic