You can take the excellent advice of all those who posted before this or you can simply forget to put it in your guitar case when you head out to a gig. For myself I find it easier to remember tunes with words, I hear the phrase and I can hear the tune that went with it. When I memorize wordless tunes I break them down into phrases and remember them phrase by phrase. Nonetheless I still often have a problem with saying "I will now play X." and then forget the first three notes and have to hum the ending to myself to get back to the beginning. When that happens I try and force myself to remember the names of the notes at the beginning ie, "Its A D D C B" not just "da Da da da daa"...