In the context of home recording and home mastering, IMO, the goal in mixing is to not need to put anything on the master buss. Anything you do on the master buss is the same as inserting the same effect with the same settings on every single channel, not something you'd ever think about doing.
The goal in mastering is to arrange the songs for a project in the correct order and spacing, and make them sound coheisive, level and EQ'wise, and set the PQ codes and such for redbook.
If there is some program EQ like a Pultech that you want to use, or if you want to put a mastering compressor/limiter on the whole mix, it's a better idea to first create the mix file, then create a seperate pre-master (don't overwrite the mix file). You should end up with a set of mix files, a set of pre-master files, and a final burn file.
I like what Mshilarious said about Mastering being a concept as well as a process. You really need to take some time in between mixing and self mastering to de-couple from the project, and then you need to forget whatever you were thinking while mixing, and approach the project like it was just handed to you.