If you want a part to be "present" or "predominant" in the mix you often could use compression on that instrument during the mix.  This could be the lead vocal or lead guitar or any instrument that is critical to the tune.  

Compression settings for this type of work usually use the settings shown at the right.

Similar settings can be used for the overall mix to get its apparent (to the ear) level up.  Use smaller ratios (up to 2:1) and longer release times.   Compressors with an "overeasy" feature or "soft knee" work the best.

In digital recording there are extreme peaks that can cause the overall average level to be low.  If you are mixing down to analog tape, many of these peaks have been "rounded off" by the tape.  You can control these peaks with the LIMITING function of most compressors.  This is accomplished by setting the ratio very high (10:1 or more). According to Ben Blau of RID:

"To achieve this, engineers often seek to use very fast attack and release times with a high ratio and a hard knee. This will very quickly reduce the gain on the audio peaks, which are often not noticeable to the ear. This is quite common in mastering, since it allows mixes to be recorded much louder on digital media, such as CD’s without going into digital clipping. In other words, -6dB of peak gain reduction will allow a song to be recorded twice as loud to your ears on a CD!"