I do audio mastering as a part of my job as audio engineer for a computer games company and have mastered a number of game soundtracks, songs, EP's & albums.
It's an art and not just in making a song sound louder. It's doing the least ammount of dammage to a song, knowing when you have to apply processing (EQ, Dynamics, Limiting etc) and when you don't, being transparent with your work and being smpathetic to each song. Making all the songs work perfectly on their own and when sat with each other and allowing the album the ability to play from start to finish naturally with the right track order, the correct type of fade ins/outs and leaving the right ammount of time between each track.
What it's not is slapping a Waves L2 plugin on and squashing all the dynamics out of a track just to make it really loud.
If you can afford it go somewhere serious, somewhere that will give you listening sessions in the studio. There are also lot of online inexpensive mastering services which are really very good but becareful of the 'L2 Bedroom' mastering engineers.