Thinking about it and reminding yourself is not enough. You have to go far below the conscious level for any attempt to work.
Try one of these:http://www.footsmart.com/product.aspx?shortsku=30411&srccode=YDFR&cm_mmc=YDFR-_-CSECI-_-20070905-_-NA&ci_src=14110944&ci_sku=parentitem30411
It doesn't hold your shoulders back for you; rather, it consistently reminds you to hold your shoulders back. The first time you wear it, you might be able to go for 20 minutes before you tire out. That shows how much you need to increase the endurance--not the strength--of those muscles.
When you take it off, you'll find that holding yourself in a more upright position feels like the most natural thing in the world. But that'll wear off after a while. The next day, though, wear it again, for just a bit longer.
You can't think yourself into having good posture all day, any more than a person who doesn't work out can think himself into benching 300 pounds. You have to build the endurance in the muscles and train them to work when you're not thinking about it.