Replacing button conductive pads? (NDS Lite)

The buttons on my ds aren't nearly as responsive as they used to be and I think the conductive pads are to blame.
It's mainly the d-pad and the A-button and they don't make the clicking sound that the B,Y, and X buttons do.
My DS is also three years old, it works perfectly except for the buttons.Where's the best place to buy them from?
How hard is it to replace them?
I've never opened up a ds and my expertise with anything technical is putting together a computer.
Could it be the buttons themselves? Could the plastic on them of worn off instead?Thanks for any help and sorry for being such a noob.
Also, I'm sorry that it's more of a repairing question then a modding question.

Could be worn, or dirt/dust has accumulated underneath, but nothing lasts forever. But first word of advice if you have never taken apart a DS before and you don't feel comfortable in doing so then don't do it. You will need a "tri-wing" screwdriver for one and just slowly break it down read this it might help but don't go all crazy with it as you can easily damage it if you are not careful.

Thanks for the guide and I know I could break it.
Seeing as my A button and right on my d-pad don't work anymore now (couldn't resist playing the new games)
and nintendo charges $75 to repair it, I think I'll try repairing it instead of just buying a new ds.
Thanks again for the help. :D