It sounds like the average level of your mixes is low compared to the peaks. Please remember that it's hard to make actual numerical suggestions without actually hearing and seeing the file... AND that my personal opinion is that mastering should be left up to real "Mastering Engineers". With that.....
Compression
You could put a compressor over the mix... say starting at ~1.5:1 or 2:1, with a long release ~500 ms. Rather than "Peak Mode" put it in "RMS Mode". In RMS, the compressor works on the "average" level of the signal rather than looking for peaks to tame. This will increase the "average" level of the mix. Our ears are sensitive to average level and not peaks. Play around with the threshold until you hear the mix being compressed in the way that you want it to be. If you haven't already, start raising the "make up gain" till the mix rises in level to where you want it to be. I generally don't prefer the sound of a overly-compressed mix when the make up gain is raised. Doing this after limiting sounds better to my ears most of the time. All of these numbers are relative, because every mix is different and is mixed to different absolute peak and average levels.
Limiting
You could use a limiter to limit the peaks. Limiting can be used "in addition to" or sometimes "in place of" Compression. If Limiting and Compression are used together over a mix, people usually Compress first Then Limit After compression. Also, compression over the mix should not be very heavy (if it is then you didn't mix the stuff right). Limiting is purely for peaks, although if you "Hard Limit", you can actually start to mess with the average level of the mix. Set the limiter for a starting ratio of 15:1 or higher (depending on the mix and the desired amount of limiting). The trick with "general/moderate" limiting is to have only the peaks hit the Limiter. If you set the threshold right and this happens, you will essentially push the peaks down to a level "close to" or "equal to" the average level of the rest of the mix. Thus, you will have made the overall level of the mix closer to being "even" and "under control".
Now that's 75% of it. Once you do this with the limiter, you can then raise the overall "Post Limiting" level of the mix so that the mix "peaks" at 0 dB or a little less than 0 dB (I generally go for 0.05 - 0.1 because some CD players are calibrated differently and 0 dB on once scale will cause another scale to distort, but just my opinion). Raising the "post limiting level" is the trick to having a dynamic mix, that still retains the overall natural dynamics of the unlimited mix, but that does not have stray peaks all over the place that cause havoc to the listener and don't allow you to use up the full audio scale.
In Practice
When doing a home mastering job (I stress again that real mastering should be left up to real Mastering Engineers), I generally EQ the mix before limiting, because any EQ boosts raises the overall level of the mix putting a mix that was brought up to zero "OVER ZERO---- hence DISTORTion!). The converse is true, any EQ cuts lowers the overall level of the mix leaving you with a less-than-optimal mix, compared to the same mix brought right up to zero.
Also, I generally try to get the dynamics of the individual instruments under control IN THE MIX (before attempting a home mastering job). The most common out-of-control signals are bass, drums... esp kick and snare, and vocals. Get these straight in the mix stage so that you don't have to resort to multiband compression and other wild stuff in the mastering stage. Most of these tricks should be used to rescue bad mixes. It's also true that most of those problems could be solved simply with better mixing. If you are boosting, cutting, compressing a whole lot (I know that's a relative statement), then you probably did not mix the track right to begin with. Good mastering is subtle and not heavy-handed.
Signing Off,
Rev E
P.S. You don't need a $10,000 Compressor or Limiter to do this! The expensive stuff simply does the job in a way that sounds more pleasing to our ears.