While it’s sometimes considered bad practice, there’s no reason you can’t serve as your own mastering engineer. This article from Music Tech Magazine covers (in flashy detail) everything you need to know about mastering your own material:
One of the reasons people take their music to professional mastering engineers is that they bring a ‘fresh pair of ears’ to a project. If you’ve been even partly responsible for writing, arranging, recording and mixing your material, your judgement mat be coloured by over-familiarity with its contents. So have somebody sit in on the mastering session who’s never heard a beat of it before.
The article covers all the essentials, from EQ and A/Bing your material against a good reference, to software and hardware choices. But I’m particularly fond of the advice quoted above. In fact, even if you’re not going to bother mastering a track, it’s still helpful to have a trusted outsider give you feedback on your mix.