I use reaper for live effects all the time, so believe me, I feel your pain.
Seafroggy's got the high points. Decrease your buffer size, and you'll have less latency (though you'll tend to have more problems with skips and glitches if your computer can't keep up.)
Get an interface with hardware monitoring for even less latency (and it's less work for your computer, so you'll get fewer hiccups.)
Another trick that I've found in reaper is that if a track isn't armed for recording, Reaper buffers it on playback to save cycles. If your channel is going through any other tracks to pick up effects, you'll want to arm those ones are armed.