[The Mastering House Inc.]

Assuming that your peaks are close to 0 dBFS, the primary way of increased perceived volume is to reduce the dynamic range and increase the level overall.

First look for stray peaks in the audio. Reducing them will cause the least damage overall to your sound since they are usually short in duration. A brickwall limiter is the main tool for doing this, though some engineers overload converters, or simply raise the level of the audio into clipping and then reduce the volume slightly to avoid clipping (shred, not recommended).

In addition to lowering peaks, most MEs will also use compressors to help raise the average level. Often times we use more than one compressor to help "spread out the load" and for different purposes. Some just due to the character of the sound of the compressor, other times using one possibly for quicker response, another for slower, etc.

In addition to comps and limiters, there are of course things like volume automation and getting out the old pencil tool in some extreme cases.

Perceived "loudness" is more than just dynamic manipulation however. It's a combination of frequency balance, average levels, and generally a great mix.