Head position could affect your trim, so if you're head is too far "up" then your feet are going to be... anyone? Bueller?
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Keeping my hands outreached in front of me and my head looking down also do not do enough to counteract my sinking feet. It helps, but not nearly enough.
I agree with everyone else regarding shifting some weight from your belt to your cam bands/tank valve.
In addition, the question of posture in the water is critical. You don't mention your propulsion technique, but if you are using frog kicks, your resting position should put your feet above your hips, which changes the fulcrum/pivot point of your body in comparison to holding your legs straight out behind you. If you are doing this and your knees are still sinking, perhaps you aren't arching your lower back enough. Many of my students have complained of lower back tension after working on frog kicking because of this unaccustomed position--they are used to stretching out in the water and doing flutter kicks.
3 sets of fins: Hollis F1 Batfins (L), ScubaPro Jet Sport Fins (Blade, XL), Zeagle Talon (Blade, XL)