I don't really agree with your reason for wanting to lower your mmr, but when I feel compelled to do it, I do so by way of learning new champs/roles (which is usually part of my reason for wanting to lower my mmr in the first place). In this manner I can legitimately be trying my best to win, but I'll probably still lose >50% of the time, and hence lower my MMR. (Or I turn out to be a natural on the champ and suddenly I have a slightly bigger champion pool.)
I am assuming you're referring to normals though instead of ranked. If you don't want to play seriously with the goal of facing harder opponents, don't play ranked in the first place. In normals people are usually much more forgiving of players sucking, and more open to silly stuff.
If you have friends in a similar boat, you can play normals in a premade with them, and if it's a 5 man you can pretty much do whatever (even intentionally lose, but that's no fun), and then if you're about to win (after what was hopefully a fun balanced game) then you can surrender. Your comrades won't care or will even be happy to lose the mmr, your opponents will be happy for the win, and everyone gets what they want.
That said, if you play enough games in your normal fashion, you will eventually settle down at your 'true' mmr. If you're a burden to your team currently and your mmr is higher than it should be, it'll fall just from your natural play. Otherwise, you're probably not as much of a burden to your team as you think you are, and might just be tilted from a few rough games, even if you are at or below your 'true' mmr.
