The only tool money can't buy...
You can have a million dollar studio but if you don't develop your ear for mastering all that fancy
equipment means nothing. There is no magic plugin or magic piece of hardware that will churn out an amazing master for you. Remember, mastering is the last critical step before people will listen to your music. Our ears are the one tool that is the most important. If we can't hear we can't correctly
master. Now there's not really much one can do to develop an ear for mastering in fact there's nothing special about a mastering engineer's ears. The only difference is that a mastering engineer knows what music should sound like in his mastering studio. This takes experience and doesn't happen over night. Remember that when we listen to anything we very rarely are hearing the actual source. We hear the rooms or environment's version of the source. Knowing how the source sounds in the environment is key. Now of coarse having a proper room will eliminate trial and error giving you the actual source audio. Mastering is all about listening and what you hear not fancy mastering plugins or hardware. Learn the listening aspect first and the rest will come over time.