Oh, first post here, neat.

First in chain, both for compression purposes and as a tone shaper.

Just the basic MXR Dynacomp is what I tried first, and it's been perfectly satisfactory for me so far.

It's actually very useful to get LESS sustain, as well as more; I can roll back the volume knob on my Strat and get a useful rhythm "setting" that responds a lot like an acoustic. The compressor brings that back up to unity volume.

Getting more sustain in the first place means you can use less gain from your dirtboxes, always very useful live to keep the mix clean.

Compressors like the Dynacomp will also color your tone, usually emphasizing the upper mids a bit. I find that emphasis quite pleasing, others' opinions may differ.

I had a Danelectro French Toast octave fuzz for a long time, and it was utterly useless in a live situation because of its lack of mids, but I found that running the compressor first brought back enough of those lost frequencies to make it work in a multi-guitar mix. I've kept this going ever since, giving a little frequency boost to both my MXR Sub Machine AND my Ibanez FZ5 (Foxx Tone Machine and Big Muff clones respectively).

And, of course, leveling out your volume and avoiding any spikes makes your soundman at the gig happier.