My guitars plug into my Boss GT-10. The GT-10 plugs (via SPDIF) to my Focusrite Saffire and that, in turn, connects to my computer via Firewire.
The control software for the Saffire has a little button for sample rate which pops up a tiny window:
That little EXT at the bottom locks the Saffire's clock with the digital in (SPDIF). Seems to be important (and of course, I didn't have it set before).
Additionally, using Audio/MIDI Control (Mac stuff) I selected "Device" for the Clock Source (per an Apple support web page). And finally I set Ableton to record 24-bit by default (I theorized that this would remove the need to downsample audio its way to the disk).
These new settings allowed me to quarter my sample buffer setting. Previously, I could only set it as low as 256 and often ran at 512 when the tracks started piling up. Now I can set it between 64 and 128. This reduces my round trip latency to 11ms in the worst case (down from over 30ms).