Software compression isn't necessarily a bad thing since part of the problem with modems (on top of the high latency) is the low bandwidth. The latency overhead from s/w compression is very low and will not effect ping greatly.
The most important tweak you can do, is to get a decent modem in the first place. ISA modems such as the Pace 56k tended to be the best and this is the only modem I know capable of sub-85ms pings (ignoring impractical tweaks like disabling error correction). However, most computers with ISA slots tend to be too slow for modern games so the next best option is probably an external one, the Diamond mentioned below were quite highly regarded and could ping around 100ms.
PCI modems aren't inherently bad, it's just that the vast majority tend to be software 'winmodems'. Pure Hardware PCI modems are OK.
To be fair though I haven't gamed on modem for about 4.5 years now, so the modem market may have changed now. Not sure what v92 modems are like, they might be worthwhile due to the increased upload speed.
ISP also plays quite an important role, due to the wide range of different kit various ISPs use. This can make a difference of up to 30ms, for example with my best isp I pinged around 83-90ms, compared to say another one giving 110+.
There are various connection strings you can use as well, for example people with a poor phone line would sometimes limit the speed at which they would connect. This ensured that the line wouldn't re-train midgame causing a huge lockup. So if for example, your line was capable of 48000 speed, you might limit the connection speed to 46667 instead.
If you are serious about gaming you should look at getting broadband, or if that isn't available in your area, then ISDN. ISDN is capable of sub-20ms pings which is pretty decent albeit not quite up to cable/dsl standard.