If I had the money for it, I'd probably buy one used pair in a size down to try on regularly. I have a hard time with jeans because i have a big butt and really can only buy curvy ones (otherwise I have to go up a number of sizes and end up getting them altered anyways!) so you never know if the jeans will fit for sure... that's why I'd buy used one. Then you wouldn't feel as bad if the fit isn't perfect (even if the size is correct).
However, you need to decide if it's motivating for me. Sometimes having small clothes are motivating for me, sometimes not. Depends on my move.
I have done it a few times when I find something cute on clearance, but it's hit or miss whether they will look good once they actually fit. So don't spend more than you are willing to waste I don't find it particularly motivating (or discouraging), it's just another way to measure.
However, if you find a cut/style of something you really love, buy another in the next size down. I went to buy new work slacks only to find they had changed them and I don't like them anymore
Before I was married, I had a roommate. We were very similar sizes (same height, within 10lbs of each other) and we had this pair of size 10 pink cords from The Limited hung over the bannister. We were racing to see who could fit into them first.
Now it is funny that I found my size 12 self fat. But I made it into those size 10s, and even as far down as an 8, before I got married, had babies and ballooned.
So my original comment was going to suggest not doing it. I bought a really cute skirt about 5 sizes down as motivation 7 years ago and I'm still at the same weight 7 years later. Additionally now it just sits in my closet as a reminder of what I considered failure. I think that for me I don't want it staring me in the face making me feel discouraged if I can't get there fast enough.
However, when I read a few comments up above about buying a size down, I realized that that might work for me. I like the one size down idea because it's something that ideally you could attain faster than a few sizes down. You'll have a better idea on how you'll look in them now. And, best of all, you can try them on every week and check your progress but not get too discouraged if you're not getting there fast enough. For me baby steps work better and are less overwhelming. I might try this suggestion.
I hate wearing baggy clothes, so I buy a mini wardrobe for almost every size (a few pairs of jeans, a few tops, a few skirts, etc). There are some really excellent thrift shops in my city, even a few plus size thrift shops, and most of them have half price days once a month (I got a gorgeous brand new Ann Taylor 100% silk skirt for 50 cents recently), so I never spend more $20 at each size. And I also saved some of my smaller stuff so I go closet shopping every 5-10 lbs.
I would, however, not buy more than one size smaller than you currently are. Your shape may change and those clothes may not fit right when you get there. I would be so disappointed if I worked hard to lose 3 sizes to fit into a super cute pair of jeans or skirt only to find it doesn't look good on my body shape.
I buy pants when they are on sale. I usually get a size smaller so I have something to work into. I don't go lower then that. I try them on and get them if I like them and if they almost fit. It 's fun to do this in a size smaller then you wear but I wouldn't go too much smaller because you don't know what season it will be when you are that size.
It is also fun to go to thrift stores and shop. I like that. I didn't used to be able to get my size in the thrift stores and now that I am smaller, I can.
i found that i needed pretty much a whole new wardrobe every 30-40 lbs because i started looking clownish
That's me. When I did this before, I didn't buy new clothes until I had lost nearly 50 pounds. And then, the only thing that prompted me was that I got a new job at a bank and my old work pants literally wouldn't stay up!
I just hate spending money on something I'm only going to able to wear for a short while. I'd rather wait until I'm at my goal and buy a whole new wardrobe then. But, I do have some "motivation clothes" because a few things I bought right before I quit last time are far from fitting now. But they weren't intended to be that...
I buy too-small clothes, but only if I can actually zip them up, and they're CHEAP. I still have 1 pair of jeans, a skort and a pair of capris waiting for me, and none of them cost more than $12. It's fun to pull something out of that tote and move it to my dresser.
I'll only buy a size down pair of pants. Any more, and i'll just get depressed when I can't get them over my thighs xD...For the most part though I end up getting pieces of clothing that will last me a few sizes. Dresses have been my best friend for the past year xD. And like some people have said, pants that button and zip but are tight are great modivators. Really helps you from putting on a few pounds(although sucks when your pmsing and the bloat won't allow them to zip) -.- to expensive replacing a wardrobe every size or two.
I wouldn't - I have a closet full of small jeans that I bought for those exact reasons, haven't been able to get my butt into a pair of them and it makes me feel even worse. Now I just think that, I will not buy anything new until I reach my mini goals and it definitely makes the hard work pay off
I don't find it particularly motivating (or discouraging), it's just another way to measure. 
