Quality over quantity, but we like to see at least 128 samples.
Export as 24 bit 44.1K samples
No clipping (but you already knew that)
When bouncing loops, set the loop markers or start/end points so that the loop keeps the any reverb/delay tail. Basically, set your loop, then move the end point back until the entire tail is within the loop markers.
Any tempo based content (drum loops or sounds with rhythmic delays or fx) should contain the BPM in filename.
Export Settings in Ableton
When bouncing samples within Ableton (ex: flattening, consolidating), make sure the recording bit rate is set to 24bit. (Preferences> Record Warp Launch > Bit Depth)
If using Ableton leave .ASD files in the pack if possible. (When exporting audio, enable the "Create Analysis File" button. This will automatically create a proper .ASD file for whatever you are bouncing.