How do you buy gifts for your kids/grand kids?  

HOw do you decide what to buy your child or grand child for Christmas or their birthday? I mean, do you just ask them what they want and go buy it? Do you buy them what you want them to have, think they should have? Do you buy just what is the popular toy/item at the time?

My 7 yo old GD made me think about this. SHe has been stressing about some other things, and to distract her a little I asked her about Santa, did she write to him, what did she ask him for. She said she did write a letter and asked for a Baby Alive. That was all?, I asked. She said "yes. I have been asking for a Baby Alive for christmas, this year, I asked for it last christmas and for my birthday and no one will get me one. They keep buying me video games nad movies and things, but all I really want is a Baby Alive doll."
Okay, so you now know what I am getting her for christmas, but I know her mom is getting her a Wii, and daddy, he just spends a ton of money on things like a quad and dirt bike and air hockey tables nad more expensive things that is really waht he likes. Mom tries to keep up with the times, ergo the wii. Basically they buy her what they want her to have that is also in their interests. By February she doesn't even remember what she got for christmas, but seem she remembers what she didn't get.
So... how do you buy for your kids?