Once you have exported your mix and started the mastering process, you may find that the impact of you track after any build ups or breakdowns may be slightly lacking. Of course you can go back to your mix to rectify this but often there is a quick fix that can work.
Import Your Finished Mix Into Your DAW
Grab your exported mix and import it into a fresh project. It's a great idea to start with a blank template so that you have room to work and use of your entire CPU. I find treating the mastering process as a different stage of production also gives you a fresh perspective on the project.
The mix is imported.
Apply Automation to the Affected Area
Now identify the area that needs a boost. Pay special attention to areas immediately after breakdowns, build ups and drops. You may notice that they lack impact and need an extra push.
Now use volume automation to lower the section before the critical point of impact and create a small ramp. You can afford to make this pretty exaggerated, as dynamics processing during the mastering stage will generally smooth this out somewhat.
Experiment with different curves and fade lengths here. You are really trying to create a 'transparent' effect so the listener is not aware of any massive changes in level. They certainly shouldn't hear the fade at all. If you can hear it, keep on working!
Repeat this process for every section in your song that needs it. And ensure all the 'higher and 'lower'' level parts are pretty much the same throughout the project. Usually an increase of around 5 or 6 dB should really help.
The automation is applied to the appropriate area
... a zoomed out view
Apply Your Final Mastering Treatment
When your done you can master your track as you normally would. If you find the contrast you have created is not great enough you can always go back and change your automation data until your satisfied.
Really try and focus on not applying too much dynamics processing to your track here. Not only will this greatly reduce your dynamic range and suck the life out of your track but the work you have done with your automation will simply be lost.
The master is now exported.