One of the things which definitely improve online tipping is making it clear what it will be used for. Making it easier and more visible also helps.

This leads to the question if online tipping is a luxury good. Tipping in restaurants for instance is so ingrained both socially and because the staff need the money that tips may be squeezed in an economic downturn (see http://waiterrant.net/?p=436), but they will hold up. For online content, that remains to be seen, but we can definitely try.