Get electrical tape in different colours...
... and wrap two turns of tape around the body of the jack.
Not very. Want to try ? Quick ! Where do inputs 1 and 2 come from ?
It helps a bit if you use one colour per instrument, E.G. red for guitar, orange for synth #1, etc.
This is http://www.teaser.fr/~amajorel/patch/, last modified AYM 2008-03-03