Keeping themes as DRY (“Don’t Repeat Yourself”) as possible makes themes easier to update, tweak and maintain.  When a client requests a change to an element that’s used throughout their site, a dry theme gives you the ability to change one piece of code and be done.  The basic mixins I use in all of my themes include mixins for fonts, colors, form inputs and grid styles.  Once those are in place, I also create a mixin for any design elements used throughout the site—for example, a special treatment for titles.

This article called Drying Out Your Sass Mixins from A List Apart gives a good overview to get you started.  How do you keep your themes DRY?