If you want to BOOST the volume of your mix, use a multiband mastering plugin or a mastering limiter, like regebro said.
A multiband compressor/limiter lets you tweak the EQ of the mix as well as boosting the volume, and it can give a commercial "radio" feel if that's what you want. Multiband tools can boost volume by 6dB or more, but as mentioned, don't do that unless you really need to. Mixes which have been given a "full-on multiband treatment" can be quite fatiguing to listen to. Also, if your mix is destined for radio, they will push it through a broadcast limiter, which will squash your mix even further and make it sound terrible.
A normal (wideband) brickwall limiter is the best choice if you just want to get rid of the clipping, or gently boost the mix by a few dB.
Place the limiter plugin at the end of the master effect chain in your software, then set the output level to -0.1dB and the release to about 50ms. This will get rid of the clipping without changing the overall volume of the mix.
(Why -0.1dB? Because some cheaper CD players distort at 0dB, so it's usual to turn the maximum level down a fraction. This only applies if you're burning to CD - mp3s can go up to 0dB without a problem).
Of course, we're assuming your software actually supports plugins! What software do you use?
Dave Brown
www.db-audioware.com
BTW, feel free to check out our own
dB-L mastering limiter and
dB-M multiband limiter plugins if you're interested.