Add compression to the kick drum. In most live situations, a compressor used on the kick drum is a good idea as it adds consistency to the hits. Most rock and metal bands heavily compress the kick drum. Set the threshold low and the ratio of compression placed on the signal above that threshold to a higher setting.
Compress the vocals. Compressing vocals allows even the softest of passages to be heard over crowd noise. Compression should not be too heavy on vocals as it will accentuate unwanted stage noise and cause unnecessary feedback. Set the threshold low and the compression ratio low so that the compression sounds natural. A high ratio setting often makes the signal sound squashed, which can be a desirable effect on some signals, but not often used this way on vocals.
Route a compressor to the guitar channels on the live sound console. The guitars are often lost in the mix, so by adding a compressor, they will "pop" and sound even and consistent throughout the performance. Set the compressor somewhere in between the settings used for the vocals and the kick drum. This will allow the softer guitar parts to maintain their dynamics, but be loud and clear through the PA system. When the guitars are played loudly, they will not be overpowering.
Squash the bass guitar sound with a compressor in a live setting. The bass guitar is a rhythm instrument that most commonly uses compression. This give the bass a consistent sound as dynamics are often less important when working in tandem with the drums. You can often use the same compressor you use for the kick drum as they often utilize the same or similar settings. If you are short on compressors in your live rig, this is your best option.
Compress the main mix by adding a subtle amount of compression to the stereo main output sliders. The compressor is often used as a limiter in this case. It eliminates any peaks in the sound that can cause hearing damage or excessive feedback. Live sound engineers often set the threshold to compress or limit any signal above 0db to an infinite ratio. Although this will squash the sound when it hits that peak, it will also help prevent feedback and help maintain an overall smooth sound for the audience.