Frost and Steketee offer some strategies for hoarders to use to declutter in Stuff. However, their second book, Buried in Treasures, is a workbook for hoarders and the family, friends, and social workers who want to help them declutter. In both books, the authors encourage the hoarder to sit down with a helper. The hoarder then begins to decided whether to keep and item or to discard it. At the beginning, each decision is very time-consuming for the hoarder, but eventually, as the hoarder declutters, s/he can spend more time decluttering with less time spent on each decision.
An earlier commenter on a post suggested that one reason hoarders don't declutter is because s/he overestimates the amount of time spent on the decluttering process. Frost and Steketee's exercises are a good way to develop a more realistic idea of how long it will take to declutter. In addition, the exercises are also a good way to build willpower.
Baumeister and Tierney devote an entire chapter in Willpower to the famed explorer Henry Stanley (of Stanley and Livingston fame). Stanley early in life came up with some habits (such as shaving daily even while trecking through a rain forest) that enabled him to lead a disciplined and successful life. While others in his party were going crazy, starving to death, or getting killed in the bush, he perservered. Baumeister and Tierney eventually concluded that you should "use your self-control to form a daily habit, and you'll produce more with less effort in the long run"(p.159).
An ingrained habit requires no decision, and thus uses no willpower. If Irene, for example, spent a month separating her mail into bill and recycle on a daily basis, she will eventually not have to think when she opens those envelopes - she will instinctively start to sort. This will conserve her willpower and let her made additonal decisions.