It's not for me. I have never done that. I feel the need to put each garment on before I purchase it to see how it would fit me. I have always had breasts (IMHO, most designers seem to forget that those are a part of female anatomy) and usually shirts or jackest were tight in the chest area. I had no idea I would lose a good couple of inches there.
Anyway, the only thing I ever did would be that I would buy jeans that are perhaps too tight (I can zip them up but I have a muffin top in them and would not be able to wear them all day long) - I live in jeans so for me it was important to have the next size down lined up. Not only that, another reason was that I didn't want to pay for expensive jeans and I like to buy mine at Winners (a Canadian version of T.J. Maxx so I could not rely on any brand - you just have to try on half of the rack till you find a pair that fits. Also, I am tall and I need at least 33", preferably 34" in inseam - they are hard to find).

I always purchased inspiration clothing.

I actually had to break myself of the habit of buying clothes that fit just a bit too tightly. It's hard to go from that to buying clothes that fit. :P

I've fit into all of my inspiration clothes. The last inspiration piece is a size 10 pair of jeans from Express. Even now, they are a potato chip away from being too tight. LOL!

I guess it depends on your personal choice and what is best for your motivation. I have a dress my DH bought for me when I was doing really well on my diet a few years ago. I was SO close to being able to fit in it, but now.. well its not even close.

SO that is my motivation. I want to wear this dress he bought for me because I told him that I loved it!

My budget and closet are too small to buy and/or keep clothes around that don't fit. The only exception is my wedding gown (which is now too big, but I'm not keeping it to wear, so that doesn't matter).

I don't use too-small clothing as a motivator. It wasn't that it couldn't be an effective dangling carrot, but it always seemed that when I did it seemed like "cheating" to buy clothes that fit NOW, so I only ever had "too big" and "too small" clothing in my closet. And by the time the motivation clothing did fit, the excitement of it fitting was pretty short-lived (not enough bang for the buck, so to speak).

I intentionally by a few shirts snug or too small just so I can work for them. I tell myself if I REALLY want it, I'll earn it, and I don't want to buy too my things my size right now because if I stay on track they'll be too big for me in a few months. I wouldn't buy anything "fitted" though because our bodies change so much.