I have an almost 3, and we are just getting into this too, so I'll be watching the responses with interest too! Also have a 6.5 yo, so that's part of where his "I wannas" come from
I think it's just a natural thing to want something you see. I don't know if they so much 'want it' as equating it with OWNING it and taking it. Our library has a toy lending library, and we borrow things there to play with and return them later, and they enjoy it while we have it, but usually within a few days, it's 'old news'. (These are NEAT toystoo--like folk art puppets and books, wooden puzzles, and wooden pull toys, trucks, etc.) But there are some all time favorites we get over and over again, or end up purchasing for our collection.
Something we've done with my older son since he was was about this age was 'the list'. Whenever he asks for something, we tell him to add to his 'list' and we'll research it. He's added tapping a finger to his temple and winking his eye, like he's 'registering' it into his brain. But, then later, when he's trying to think of things for birthdays or Christmas, we talk about 'the list' and what's on it. Most of the time, he only remembers a few items
and those are the ones we look into. Whether it's online, or we investigate them further when we go back to the store, we try to make an informed decision on what we're buying. And, we talk about why he wants it and how it might go with the toys he already has.
Of course, the in depth discussion doesn't 'work' totally with a 2.5 yo
But....we still do it! It will work eventually. And, we tell him to add it to his list also. It difuses the situation at hand *usually* and then we can talk about wanting the toy at a later date when the item of desire isn't immediately at hand and making them toy crazeee! It also usually keeps the tantrum down, because mommy didn't say no, but she didn't say yes. And, we are always free to dicuss it later on less tempting turf and when the memory of the gadget has faded a little.
I think it was in this issue of Mothering where I read about Tantrums! It had some great advice....and I loved her line about if you have to make a hasty exit, something about, "Pardon me, my associate is having techinical difficulties" or something like that. I'm definitely going to use that one later!
HTH and I'll be watching for other posts!