Don't drop the ball when they get older. Make sure that you take time to talk to them about school, their friends and what music and movies they like. Make it a point to call them at least once a week and try to take them out for a special day (basketball game, shopping or just lunch) every season. You want to give them memories and open a line of communication so that they know that if they can't talk to their parent, they can count on you.
Popular with the child or with the parents? I always send the present I know the child is longing for - tins of spray string, garish nail polish - and which the parent is not going to buy.
If you have lots of godchildren, send them a present on your own birthday instead of theirs - that way you don't have lots to remember. They will get so many presents on their actual birthday that yours will stand out!