Tip 1 - You don't know what you're doing.
Tip 2 - YOU DON'T KNOW WHAT YOU'RE DOING!
Tip 3 - Check in your multiband compressor, usually bundled in your mastering suite of your recording software. There should be some presets, often with names like Rock, Hard Rock, Metal, Jazz, Smooth Jazz, Easy Listening, Monster Thrash; stuff like that. Now, select each one and Z/B them. When you find one that makes your mix sound better. Select it and master a copy of your mix. (Always keep a copy of your mix. You might want a real mastering engineer to work on it later. you never want to give a mastering engineer a mastered mix to work with. This will give you a happy finished product without you having to screw anything up by guessing. Remember. Mastering is not mixing for a reason. You do NOT know what you are doing. The presets are great. If you don't have any (or you just can't find them) ask a friend if you can run your mix up on their system and master it with their presets. Do NOT try to fine tune anything. Always test your final out in a vehicle with a custom audio system. If it sounds good there, it should sound good anywhere. Don't try to "learn" mastering. There really is a reason those engineers get the big bucks and why the pros go to them. Good luck.