The most important mastering tool of all is your own ears, so the first order of business is to give your ears as clear a picture as possible.
Don't try to master your music after a long day of mixing has given you a case of sonic fatigue. If possible, don't listen to a work for a couple of days before you start mastering it. That way, when you start mastering you are coming at your work with a fresh set of ears.
Also, think about the sound you're going for and try to find some recordings that represent it well. If you aren't sure, listen to some recordings you love in similar styles to your song.
If possible, find more than one comparison recording so you get a broader target range. Sound variations between comparison recordings are helpful because they keep the reference point general (and more attainable) as you master.
Always check your mix carefully one last time before you start mastering. Ideally, you should listen to every instrument soloed in your song from beginning to end. For a big project, you should at least check your major submixes, instrument groups, and tracks with lots of controller data.
Check your setup
No matter how good your ears are, they won't help you if your monitoring setup can't faithfully reproduce the sound coming out of Reason. Studio monitors are designed for this purpose, and though some are better than others, this is the best (and most expensive) way to go.
Home stereo speakers and receivers are far from ideal, because they usually enhance frequency response and brands vary widely in how they do it. If you have to use stereo equipment, keep all controls off or at the middle setting.
Do everything you can to minimize interference from inside and outside your sound system. Check your signal integrity from the time it leaves the Reason hardware interface until it comes out the monitors. Any signal alteration will mask the sound you want to polish, making mastering difficult or impossible.