Balancing the fine line between wants and needs

I was thinking of this yesterday when the girls saw about 1000 Barbie commercials and suddenly NEEDED all these new Barbies, even though they have about 20 Barbies between them. They begged for them for their birthday present (we are having their party late this year, hopefully in the next few weeks). Then we went to the mall and they saw princess watches, which they suddenly NEEDED. They started begging for those for their birthday and forgot all about the Barbies they had been begging for prior. \

I started thinking-how do I teach them them to control their emotions in regards to wants vs. needs. Do I even have a handle on it?? At the mall, I saw an adorable outfit (tights and sweater-very hanna anderssonish) and I suddenly wanted it for the girls...but of couse, everything is super pricey and I came to my senses that I have FIVE children to dress and not just the girls. I really have to buy frugally this year (and every year). I do want to to get the girls some cute tights, but I have to wait until everything is on sale and that outfit will probably be gone by then-and thats okay. I have to let my emotions not get the best of myself either. No matter what the girls wear, they will look cute. It shouldn't make me feel bad that I can't buy that specific outfit. So how do I teach my kids that same lesson...especially when I am 28 years old am still trying to learn it myself?