Designate A Space For Everything.
No exceptions! Decide what goes where, and stick to that decision. For my daughter, that means every toy gets put away when she's done playing with it — leaving it on the kitchen table is a no-no. For me, I have to remember that depositing a half-read book on the couch or dumping papers on the coffee table is just clutter in the making and breaks the "designated space" rule. But once I have it in my mind that things have a right place to be, everywhere else starts to feel like the wrong place, and it gets easier to remind myself to put items away properly.