For mastering, I use my audio editor mostly. And I do it the old-fashioned way; one step at a time with the ears and brain being the final judge (and a good look at the meters).
IMO there ain't no magic mastering plug out there that can do what I can do, simply because every song and every part of every song is different. Practice. Read. Experiment. Listen. Especially to other finished works that you dig the sound of, and be analytical in your listening of them and then compare them to your work with the attitude of what's missing, what's different, what's wrong and what's right.
Listen at LOW volumes only, and on your target mixing system as much as possible.
If you don't know what Fletcher-Munson means, then start finding out now. Learn to set up your home stereo off of the Fletcher-Munson Curve at various volume levels by simply listening to it.
Learn music and listen to all different genre and kinds all the time. Take what you like and leave the rest, but again listen critically and analyze what you are hearing both in the way of instrument, performance and recording techniques. At first even using common terms like hard, soft, cold, warm, etc. will start you on the path to developing those golden ears. Eventually, if you keep up the study of the technical side of the game you will be able to use technical jargon to mean the same things.
Get your hands on a Note-to-Note Frequency Chart, and hang it on the wall in front or left or right of you in easy view. Use your guitar or keyboard or horn to pinpoint notes you hear in a mix and then look at the frequency chart. Do that long enough and you will not have to look at the chart any longer, you will listen to a mix and think, "There's a problem around 1200Hz here... or whatever. This is the key to opening up the EQ situation into something you can really use; that and learning how to Sweep an EQ to find problem spots.
Lastly, one must listen at an overly-critical level and let nothing slip by. Don't ignore any little thing and think it won't get noticed. Whatever flaw you hear or detect needs to be fixed and one needs to have the attitude that they will do whatever that takes, even if it means starting over from the first note (yup, I've done that).
Don't let anybody hear any work of yours that you know has a problem in it that needs to be worked out first. Can't emphasize this point enough. A viewing of a work should never start out with an apology for what you already know needs work. This business is rough; excuses don't get you anywhere but back out on the street. Decide that you will have no excuses in your work and do whatever it takes in the way of time, work, redos or whatever such that you don't have that problem. Besides that, even if you know there is a problem in there, don't point it out before they hear the recording: they may never notice it in the first place. Far too many ask me questions like, "What do you think of my mix, I know everything's too this or too that, but what do you think of it anyway?". Shoot, before it's played back I already know what's wrong with it because they knew what was wrong with it. Get the thing as good as you possibly can get it and then some, and THEN play it for someone. Dazzle me.
Don't expect great results the first time out or the tenth time out for that matter, but expect a gradual improvement as you learn and gain more experience. At that point it may be okay to start out using one of the easier-to-use Mastering programs or plugins, but don't let it become your crutch, take the same mix and bring it into the audio editor and see if you can duplicate or better what the plugin did for you.
Don't do just one Master. Save your original mixed down trackset separately so you can go back and have another go at the same tracks in a different Mastering Session. Try doing things in different orders, try other things, always always following your ears and not a formula. It is not uncommon to do several completely different masters of the same tracks on different days even, and then lay back and compare 'em before releasing 'the one'.
Mastering can be so much more than simply getting levels to match from song to song on a disk or bringing levels up to some certain level of SPL across the board. Often I can accentuate the intended feel of changing parts in a song, add to the tension and release the musicians were going for, make it cold and harsh, even sterile or make it lush and warm. You can, too. Let the SOUND and your inner soul guide you, and if you would like to do something and can't figure out how, pick the best possible choice and give it a try, one can always revert or undo the bad choice.
Remove the word "but" from your vocabulary. Ask questions, but don't make excuses. It is a poor carpenter who blames his equipment. He should have known he had a crooked hammer going into the job and corrected that somehow. Don't present your work with the bent nails showing, pull 'em out and redo that part.
Speed comes from practice and experience and not from short cuts. Those who are using a PC sequencer and avoiding the learning and use of the Midi language are a good example, sure they can make recordings that way, and sure they can make dynamite recordings that way, but it's the harder path to follow most of the time. Refuse to let ignorance on a subject become your bliss. Set your goal way up high and then set out to beat that mark.