Correct Posture: How to Hold a Concertina

First:  When drawing or pressing the bellows, always have keys or the air valve pressed!  Forcing the air out is how you get leaks in the seal of the bellows (besides regular aging of the bellows).

Now, I'm no expert, but just like there is good posture for holding and playing any instrument, I believe there is a correct way to hold a concertina as well. Gary Breuggen I think shows it very well: standing and playing on one knee:

 

For the hands - put your right and left hands in the appropriate straps with the thumbs on the outside, especially so that you can squeeze the air valve with your right thumb easily.  [The air valve allows you to change between fully drawn and fully pressed without making any notes or ruining your bellows' seal]

There should be two rings at the top of the bellows.  This is where straps from a shoulder harness would clip on (if you don't have them already) if you want to play the concertina standing.  See Dan Greutzmacher for an example of the shoulder straps.  Alternatively, you can use a simpler neck strap as shown on Dan Greutzmacher's site which is good for sitting on a tall chair.

When playing seated, I think it's best to hold the concertina on one (horizontal) knee.  For me, it feels better using the left knee.  I think this is because the right hand is more active, so its better to keep it more in front of your body as much as possible and not way off to the side.

When playing and drawing the bellows open, pull the sides out and the tops down (the concertina will form a smile).  When pressing bellows closed, switch the bellows into a smile and press in and down. With a good bellows seal, this allows the bellows to naturally fall open while drawing, and naturally press closed - using the help of gravity!  See this video of Jerry Minar for good bellows form, and of course those of Dan Greutzmacher as well.

Keep your fingers arched and centered in the keys as much as possible, over the 5-9 keys on the right hand side, and over the 1-4 row on the left side.  When playing distant buttons, stretch your finger to reach the key without moving your entire hand if possible (within reason of course).

The Bandoneon players have good drawing form typically - check out the guy playing this wedding: foot up on a chair and playing over his knee.  However, the tango music style allows the bandoneon player to play mostly in draw, so they don't do much pressing, as you can see from the Godfather of Bandoneon players: Astor Piazzolla.