Only Horis are licensed by Nintendo. Put them on PROPERLY. There's ways to screw this up, but if they're put on properly they should last you a while.
Several points of advice (compiled from posts here and there):
1 - Be careful not to get the screen protector inbetween the touch screen and the DS case. This will result in a 'permanent
touch' which in turn will render the touch screen useless until you properly re-apply the protector. This is one of the most
common problems for those who apply the screen protector for the first time.
2 - Get rid of most the dust particles by applying it in the bath room after the steam has 'gone' and take your time. Make
sure you put your things on a clean surface.
3 - Clean the touch screen. Check for cleanliness. Then apply the protector.
4 - Should you notice dust and hair underneath the screen protector take it off gently and remove it with tape. Re-clean the
touch screen surface with a microfiber, lint free camera or sunglasses cloth, and reapply without your fingers touching underneath it.