Declutter and Revise
Of course, the easiest way to save money on decorating is to keep from spending it in the first place. By accurately assessing what you already have, looking at the real-life uses of your rooms, and moving a few things around -- or getting rid of them altogether -- you can revitalize the whole place without spending a dime.
Simplicity and minimalism are in vogue not just for the aesthetic sensibility, which may or may not appeal to you, but because home life itself becomes increasingly complicated. Most of us could fill a closet (or several) with our unused gadgets and gifts, dusty knickknacks and all the other physical symptoms of real life. By applying a discerning eye to the surfaces and "junk drawer" areas of our homes, we can find new places for these objects: All the better if that means the items move on to resale or charity shops, or the living spaces of youngsters moving out for the first time.
Likewise, you can do a sweep of each room, thinking about new purposes for furniture and objects that aren't used in practice the way you thought they would be. That comfy chair in the bedroom -- the one you intended for reading before bed -- is only as useful as you make it. If the reality of your life's rhythms doesn't match up to the ideals you once imagined, there's no shame in moving that chair to a dead space in the den, where it will see actual use.