Stabbing with Air

[KStV]

I am a novice working on developing a tight/aggressive game. When I completely miss a flop, I don't seem to have very good instincts as to when and how I should try to steal the pot. I'd like to start making those decisions more systematically and would appreciate any insights/advice.

The basic concepts I think I understand are:
--This is highly read dependent (i.e. is Villain likely to fold?)
--This is increasingly (perhaps exponentially?) more difficult to do with each additional villain in the hand.

These have been helpful guideposts, but I'd like to take my thought process in these situations to the next level. I am interested mainly in situations in which there is almost no bet I would call and very little reason for me to remain in the hand.

I understand that the variables here are endless, so I'd like to think about four situations and allow the analysis to bring out other details:

HALF-DEAD PAINT
--Hero holds: K [img]/images/graemlins/heart.gif[/img]Q [img]/images/graemlins/heart.gif[/img]
--FLOP: A [img]/images/graemlins/club.gif[/img]A [img]/images/graemlins/spade.gif[/img]5 [img]/images/graemlins/club.gif[/img]

BACKDOOR IS LOCKED:
--Hero holds: 7 [img]/images/graemlins/heart.gif[/img]8 [img]/images/graemlins/heart.gif[/img]
--FLOP: A [img]/images/graemlins/club.gif[/img]K [img]/images/graemlins/spade.gif[/img]Q [img]/images/graemlins/diamond.gif[/img]

NOTHING IN THE MINE:
Hero holds: 4 [img]/images/graemlins/club.gif[/img]4 [img]/images/graemlins/diamond.gif[/img]
--Flop: K [img]/images/graemlins/heart.gif[/img]A [img]/images/graemlins/spade.gif[/img]K [img]/images/graemlins/club.gif[/img]

RAGFEST
--Hero holds: Q [img]/images/graemlins/heart.gif[/img]J [img]/images/graemlins/heart.gif[/img]
--FLOP: 8 [img]/images/graemlins/spade.gif[/img]5 [img]/images/graemlins/club.gif[/img]2 [img]/images/graemlins/diamond.gif[/img]

When is it a good investment to try to buy the pot? When is it likely to be just a charitable donation to the table?