A couple of compression guidelines

USEFUL COMPRESSOR SETTINGS
SOURCE	ATTACK	RELEASE	RATIO	HARD/SOFT	GAIN RED
Vocal	Fast	0.5s/Auto	2:1 - 8:1	Soft	3 - 8dB
Rock vocal	Fast	0.3s	4:1 - 10:1	Hard	5 - 15dB
Acc guitar	5 - 10ms	0.5s/Auto	5 - 10:1	Soft/Hard	5 - 12dB
Elec guitar	2 - 5ms	0.5s/Auto	8:1	Hard	5 - 15dB
Kick and snare	1 - 5ms	0.2s/Auto	5 - 10:1	Hard	5 - 15 dB
Bass	2 - 10ms	0.5s/Auto	4 - 12:1	Hard	5 - 15dB
Brass	1 - 5ms	0.3s/Auto	6 - 15:1	Hard	8 - 15dB
Mixes	Fast	0.4s/Auto	2 - 6:1	Soft	2 - 10dB (Stereo Link On)
General	Fast	0.5s/Auto	5:1	Soft	10dB

Bass	Try starting out with a ratio of 4:1, and a fast attack and medium release. I usually use the hard-knee type of compression here since bass is such an attack-oriented instrument. But if you were playing smooth jazz bass, then you may want to try soft-knee. It depends on the sound you are trying to get.
Guitar	This depends on the type of sound you are using, but a good general place to start is 2:1 on acoustic, and maybe 3:1 on distorted guitar (although you may need 4:1 here.) To get a good sustain, try a 4:1 ratio, use a fast attack and slow release. Then play the note you want to sustain, and raise the ratio until the sustain is as long as you want it.
Drums	Drum signals are often compressed due to their hard-hitting attack volumes. If nothing else, compress the snare drum, because each hit will likely peak higher than other hits. Try starting out with a ratio of 3:1, and use a fast attack and release. If the signal is still peaking, try using a ratio of 4:1. This method could also be applied to the toms. As for cymbal hits, try starting with a 2:1 ratio (moving to 3:1 if needed), using a fast attack and a slow release (to preserve the natural decay time of cymbals).
Vocals	As with drums, compression is also commonly used on vocals. The ratio to start at varies for each singer, since some may be very strong and loud singers, and others quieter, having a smaller dynamic range. Try starting out with a 2:1 ratio, with a fast attack, and medium to slow release. Keep increasing your ratio until you get your peaks under control.

Attack/Release-Time Key:
Fast = 25-50ms; Medium = 100-500ms; Slow = 1sec–2sec (all are plus or minus).
Basic Instrument Settings
Distorted guitar: Fast attack, slow release, ratio of 4:1, or higher for more sustain.
Acoustic guitar: Medium attack and release, ratio between 2:1 to 4:1.
Drums: Fast attack, fast release, ratio between 2:1 to 4:1.
Bass: This one’s tricky. To tame aggressive attacks use a fast ratio, to accentuate attacks use a medium attack with a medium-fast release (good for slap bass), to increase sustain use higher ratios with a slow release (depending on music).
Horns: This depends on what you require: for smooth background pad-like performance, a fast attack, slow release, and higher ratio (4:1 and up). A fast attack and fast release will add punch. The requirements of the music dictate here.
Piano: For lively rock piano style, use a medium attack to let the transient through (like the snare) and a medium-fast release. Ratio of 2:1 to 4:1. For ballads, use a fast attack and a slow release.
Vocals: Medium-fast attack, medium release or slow depending on music, ratio of 2:1 and no higher than 4:1. To make the vocal sit in the track more effectively, try a higher ratio with a higher threshold, or a lower ratio with a lower threshold. In either case, compression will be heavy, relatively speaking. Gain reduction can be either 2dB to 3dB at the higher ratio, or 3dB to 6dB at 2:1, provided there is little or no reduction during passages sung at normal levels.

ELECTRIC GUITAR T: -1dB; R: (Betw.) 2:1 and 3:1; A: 25ms to 30ms; R ~200ms; G adjust I/O equally
ELECTRIC BASS T: -4dB; R: (Betw.) 2.5:1 and 3:1; A: 40ms to 50ms; R ~180ms; G adjust I/O equally
ACOUSTIC GUITAR T: -6dB; R: (Betw.) 3:1 and 4:1; A: 150ms; R ~400ms; G adjust I/O equally

VOCALS
(to even out vocal performance)
T: -8dB; R: (Betw.) 1.5:1 and 2:1; A: <1ms; R ~40ms; G adjust I/O equally
(to bring vox way out front in mix)
T: -2dB; R: (Betw.) 4:1 and 6:1; A: <1ms; R ~40ms; G adjust I/O equally