For some people buying clothes ahead can be great motivation. For myself, it hasn't worked very well in the past. That's partly because I would get frustrated and also because a lot of the styles I liked and bought of smaller clothes didn't actually flatter my body as it changed as I became smaller. If its an item you really don't think you'll have another opportunity to buy though and think you would really regret not buying it now, then I say go for it and hang it up somewhere it sight for motivation