h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

I've been playing blues harmonica for two or three years now including a stint in 2003 in a regularly working blues band. From my own personal experience, I'd humbly add the following to the list:

DO buy your kids their own harmonicas so that there's no excuse for them playing yours. There's nothing worse than picking up your harp and discovering - too late - that it has (for example) a sticky orange flavour.

DO try to break your harps in by playing them gently-to-moderate - without bending any notes - for a couple of hours. I've lost too many harps to early graves, through bent/broken reeds, through lack of breaking them in properly.

DO be assertive in a live performance situation and make sure that your sound levels are UP relative to the vocals, guitar and the rest of the band. You don't want to blow the audience's eardrums out, but too often you'll be the victim of a bad mix and/or the rest of the band's levels just knocking you out altogether. Good harmonica playing shouldn't dominate the rest of the band, but it should be HEARD.

DON'T use too much tongue when you're doing tongue blocks (ie playing blow or draw chords in which you're "blocking" the passage of air through certain holes with your tongue). Too much tongue and the harp quickly blocks up with saliva with horrible results. My harp teacher often tells me to just touch the harp gently with my tongue "as though the harp is on fire".

DON'T buy a large number of any given brand/make of harmonica until you're absolutely sure you like it. I once bought 7 Hohner Pro Harps only to discover that the protruding reed plates cut my mouth, lips and gums to bits. Instead, just by one (say an A or a C), try it out, and if you like it, buy more.