For me, we quite simply had to be 100% strict with ourselves, because it's just too much of a slippery slope if we're not.
Honestly, I would be embarrassed to show the pics to anyone else, but we took before and after photos of the house and garden and it's just amazing now. We made it so that there was nothing on the floor in the bedroom, bathroom or living room, and nothing cluttering up workspace in the kitchen.
But I have had to become SO strict with it. For example when DP leaves his socks on the floor and goes to get into bed, I tell him to take them to the laundry basket in the bathroom. It might sound stupid, but you leave a pair of socks there, or you put your books on the bedside table instead of in the drawer, you leave a water bottle by the side of the bed .. and before you know it you're back to square 1. Admittedly I barely have to pass comment now, as things do become habit in the end.
It's the same in the kitchen. I'm adamant that DSS does the dishes STRAIGHT after dinner, rather than wandering off and saying he'll do them later, because before you know it it's bed time, they haven't been done, and they're cluttering the worktops. Then the day after more dishes get added, and if there's dishes everywhere it looks messy anyway so there's little motivation to make sure you put things back in cupboards when they've been used etc..
Gradually it does get easier, and making a schedule for things like hoovering and cleaning is a must.
Honestly I am shocked now to think of the way we used to live even 6 months ago. But now we're slowly decorating the whole house too and that helps in itself, as I think you take a much greater pride in things when they're nice and new!