[TaylorAlexander]

Or feed the marker's liquid reservoir from a larger tank.

[theOnliest]

Or use a chalkboard.

[guelo]

Chalk would still run out and need to be replaced. Some kind of sand drawing would be more of an infinite "ink".

[TaylorAlexander]

Hilariously, a good way to do this with infinite "ink" would be to draw the letters on a touch screen with a stylus.

[acmecorps]

Or, just use one of the children's magnetic drawing board?

http://en.wikipedia.org/wiki/Magna_Doodle

[jagger27]

Or some gears and a battery.