Memorize, memorize, memorize. Except in an orchestral setting, where, as Robin points out, the maestro is telling the players how to "feel" the music, IMHO having paper on stage while performing is tacky. Others disagree with me; but that's OK.
It also, in part depends on how long the piece is, too. Memorizing a major concerto or opera or whatever - say The Planets Symphony -- is not realistic. But with dulcimer we're mostly playing repetitions of a verse and chorus.
I think its possible to have a memorized repertoire of over a hundred songs. I know I have more than a hundred songs that I can play with little or no prodding. I do keep a cheat sheet around with the first few measures of over a hundred songs on it, so I can remember how a tune starts when I'm hearing all sorts of other music around me at a jam or open mic. Again it depends on the nature and complexity of the tunes. I play mostly folk tunes with a verse and chorus; Robin plays more complex fiddle tunes.