Another good method is to not buy anything or set anything up or whatever if it requires an additional action. The liklihood of you taking that action is slim to none.
Example: you buy an awesome nightstand that would be awesome with a fresh coat of paint for $40 at a garage sale. Reality: You don't paint it and now you have a worthless ugly nightstand that you will neither use, paint, or sell. Yes the nightstand was only $40. But you would actually be better off spending $60 or even $80 for a nightstand that you get use out of. If you buy it, make sure to stop at Home Depot on the way home, buy the paint, and paint it before the days end. That is the only way this situation would work.
Example: You clean your room and make big plans for getting shelves and putting all your little trinkets and knick knacks on them. Until then, you now have a fat pile of trinkets with no home needlessly cluttering your desk or bed or whatever.
Only do and buy things that you can actually use TODAY without further action, unless you are a sincerely dedicated person who actually paints things. A good deal isn't a good deal if you never use the thing. Similarly, just make changes to your room after you buy the shelf, not before.