I think the front seat brace position is to sit with your back pressed as hard as you can against the seat back with your hands on the seat either side of your legs and your feet flat on the floor away from the pedals. You then brace yourself with your arms and press down with your feet. No idea where I saw that.
Also read that stuffing something like a folded-up jacket on your head can help reduce the knock if you do hit the things in front of you, although quite how you do that practically whilst using both hands to brace I don't know!
As an aside... When I took my 6 year old son over the channel I bought and wore a helmet on the basis that I didn't want to end up knocking myself out and leave a child to fend for himself! Only put the thing on for the water crossing. I try to keep it out of sight at the airfield to avoid having people give me funny looks!