I'm in the midst of a 12 item year and it has been great. I haven't necessarily stuck to one-per-month, and if I keep the few items that I've picked up over the past couple weeks, I'm left with only one more purchase for the year, but I really do see myself succeeding with my goal by December. I have noticed that I"m wanting to shop for the upcoming season and get it sorted out rather than purchase one thing per month continually through the year, but that's okay. At least once fall/winter is complete, I can shut my shopping brain off and concentrate on creating outfits from what I have.
I don't think you could do this well if you've got a closet full of clothes that you're not satisfied with, but when I started the year, I had done quite a bit of shopping over the past couple years and had pretty much a completely overhauled wardrobe. Not much in my closet was over 2 years old. I don't know that I could sustain a 12 item per year schedule for more than a couple years though... I think sooner or later my clothes would be wearing out at a faster rate than I would be replacing them. Does that make sense?
I will say this: I think the biggest reason I have been so successful this year (and I realize that the year is barely half over) is that I had hammered down a wardrobe size I was comfortable with and every category of clothing was complete. I also imposed on myself a one-in-one out policy. In order to buy a new pair of jeans, I have to have a pair that is needing to be set out to pasture. This keeps me from making impulsive purchases. I don't have a pair of jeans that need to go, so I don't need to buy any more. End of discussion I had 2 cardigans that really had seen their better days, so my goal was to replace those 2 cardigans and then not mess with my cardigan collection again until next year.
The other thing I'm trying to be mindful of is keeping a variety of "dressiness" levels within each category. For example, I have 17 dresses. 5 of them are winterish, but of the 12 summer dresses, 5 are super casual (as in summer version of jeans and tee) an the other 7 are a bit dressier (things I wear to church, parties, etc). I replace casual with casual and dressy with dressy. I think this is important because otherwise you might fill your long sleeve top category with all dressy silk blouses and then never feel that you have anything to wear when the occasion calls for super casual.
I'm thinking that next year, I'll allow myself 13 items: my slogan will be "Lucky 13 in 2013!"
oh, and I should add, that my 12 purchases this year have been "fashion" purchases. I've also allowed myself one per month of non-fashion items: bras, workout clothes, running shoes, nightgowns, etc. Socks and tights/hose are purchased as needed