Ctrl

Alright heavies, listen up.  I see many people complain about getting knocked back by sentries, which is a major problem when attempting to destroy them while uber'd.  Have faith, brethren, for there is the ctrl key.  Yes, though not completely solving the problem, crouch low to the ground, and you may get that extra second required to fully destroy the sentry or at least the engineer repairing it.  Your main weakness here is a soldier's rocket or demo's grenade, which, though also dampened, can still knock you back.

For taking out the hardest sentries, jump out in front of it, crouching in midair, so that you have a bit of room to get knocked back and still destroy it, timing your medic's uber at the top of your jump, for maximum efficiency.

Also while on D, hit that ctrl key to lessen the size of your hitbox, since you'll likely not be dodging much against speedy attackers.

Also:
Swapping the C button with the CTRL button can make for easier access to crouching, for those who find CTRL way out of the way like I do.     

I use ESDF, and I re-bound "Crouch" to the "Shift" key.

Probably easier to use "Shift" with WASD as well.

Probably, though i wouldn't know because i use an... "interesting" control scheme (the ctrl button is not my crouch, just figured everyone else would know what i'm talking about).

don't forget you can use crouch to hide behind things.
Yes, HIDE.

As a heavy.

Trust me, it works wonders.
Popping up out from behind a rock with your minigun revved, and just blowing away a group of enemy soldiers in a split second, then poofing back behind the rock == priceless

tru tru but how can a big lump of fat like a heavy HIDE?

You'd be surprised how many times people will just walk past you if their totally focused on where they are going.