Here's a compressor design from MicTester. Here's what he had to say about it over on FSB:
Do you remember the old-fashioned way of doing bass compression? It used an LDR across the volume control illuminated by an incandescent lamp across the speaker terminals! It was a simple method back in the 60s....
This compressor is almost that simple! It uses a handful of cheap components (I built one for £9 including hardware!), but works really well. Guitarists who've tried it often ask "Is it on?", but miss it when it's off! It's quiet, and has good "squeeze" without the gain-change artifacts you frequently get with transconductance amplifiers or FETs.
The LDR should be ~1M in the dark, but almost anything will do.
Some of the component values are quite carefully chosen. The input capacitor (15nF - made up of a 10 and a 4.7 if the 15 is unavailable) is chosen to give some frequency shaping, and the interstage capacitor prevents handling noise affecting the compression. Green LEDs are chosen because they most closely match the response of the LDR, but yellow works almost as well. Obviously, the LDR / LED combination should be facing each other, and must be in a light-tight enclosure.
I've set up the board with pads for both 10nF and 4.7nF in parallel if you don't have a 15nF cap.