My boys are like this, aged 12 & 15, christmas gets harder and harder each year, last year i got them new bag, wallet, quilt cover set, deodrant gift sets, socks, boxers, trainers,
They also like gadgety stuff, got ds2 a talking ATM money bank a couple of years ago, but i broke it so replaced it last year with a digital money jar that counts money as you put it in. Hawkins was good for that kind of stuff also some fun stuff on iwantoneofthose.com, i can usually find something they like
I also get eldest the guiness book of records every year. He loves top gear/cars so any books, dvds to do with them, they usually get a comedy dvd each as well
Are there any clothes that they want that you wouldn't otherwise buy ie a jumper or jeans that maybe cost a little more than you would usually pay.
It sort of makes it special if they know you wouldn't buy it otherwise.
If i think of anything else will let you know 