If you use a headband mount best option is to make a basic chin strap which will make sure it stays on your head, you can also put it under your hood but I prefer to dive without one unless its realy cold.
Here are some pics of my chin strap, its just an old bag strap that I have used threading the strap through the metal buckles.
I wear it more on the neck but its not very tight or restrictive but you may prefer it more on the chin.
For handheld use I use an old torch leash and just clip this to one of the rings on my BCD. Here is a pic of an old handheld setup I use for some comparison shots which works ok but im soon going to try a proper tray with 2 handles which should be even more stable.
This leash has a corded loop on the end and can be attatched around the Gopro mount itself or wound through any holes, it has a plastic clip that shortens the leash or unclipped gives you plenty of leash to be unrestrictive to wherever you want to hold it.
Head mounting is a pretty good option if you just want to dive and record what you see, its pretty cool to film entire dives like this one which was in fairly poor vis but pretty cool still. 42 min at fishrock boat to boat with a few sharks around
[video=vimeo;23553871]http://vimeo.com/23553871[/video]
Here is a shorter one in better vis just with sharks
[video=youtube;T3jZFzpG3ts]http://www.youtube.com/watch?v=T3jZFzpG3ts[/video]
Heres one you couldn't shoot any other way, scalloping as you need both hands free.
[video=youtube;gorv4r9mGxA]http://www.youtube.com/watch?v=gorv4r9mGxA[/video]
These were all shot with the EOM housing before I got the Blurfix and filters so colours aren't great in them but shows head mounting can be ok some times. I have a few Gopros now including a HD2 and another on its way so I will probably keep one on my head for all dives and another one or more on a tray setup.
Head mounting needs 60p mode to work well as the 1080 modes need a very stable cam and the higher fov also gives you more chance of getting what your looking at. 60P 170fov does work fairly well but bubbles can get in the way if your not moving or upright and this can be avoided with an extended headband mount like this.
But I didn't use it for any of these videos so bubbles do get in the way at times. It does look even stupider then a basic headband mount lol which is pretty dorky looking on its own.