Forehead against the seatback in front with hands behind/across the back of your head, holding it in place.
And the hands on the back of the head to protect against stuff moving forward from behind, and don't interlock your fingers. That way you may have one good hand to undo your seatbelt.
This of course varies from the US position which is to form a pillow with your arms and lean your head against that. Unfortunately recent studies (2015 by the FAA) have shown that with the latest Y seats the US position results in a much worse injury risk.
(For the technically minded seats broadly sit in three types: Fixed-back, Folding-backs and energy attenuating folding-back (EA-FB). Anything large built after about 2009 will have the EA-FB and a lot of post 1988 ish build and retrofit stuff will be to that standard.)
It seams with the arms as a pillow they push the EA-FB seat back away as the aircraft slows (at 17G in the tests) and thus the head can flail forward and hit the seat unrestrained.
The suggestion is that (and it's a US study looking at the US position) the brace position changes to forehead on seat in-front and hands by your side. However, the study was not looking at debris throw so I'd stick with the "British" methodology for now.
Also worry about your leg position as broken femurs makes it tricky to run away. Feet firmly on the floor. If you have super short legs and can't touch the floor when seated, consider big shoes!
Other trivia: when told to brace also make sure the tray table is properly locked in position as the tests show it really really hurts if it deploys and gets you in the neck as you're flailing forward.
at 6'5" with a relatively long torso
The brace position is 'designed' to give the best benefit to the most people. If you're outside the "normal" range then er ... sorry? Take comfort that your chances of being involved in anything requiring a 16G deceleration are tiny.