Any advice on what type of holding everybody would CALL an all in with heads up in an average situation say;

Where the 2 players have similar stacks about 5000 each or 6000 v 4000 (blinds @ 300/600 or 500/1000) and the outcome of showdown will have a huge bearing on the winner obviously.
Also where I'm either up or down 7000v3000, do you play tighter when ur leading and looser when chasing? Or does it depend on ur cards or read on opp? All I've read says things like attack ur opps blinds like a mad man etc but going all in every other hand will just get me caught out and feeling like a prick when my 8-3 etc comes up against a real hand will it not?

This is one of my weaknesses I'm sure, I always seem to question myself whichever way I go. It just seems like heads up u gotta be lucky and I don't like the idea of that, obviously I'd like some sort of advantage!
While I'm on the subject how strong a hand do you reccomend checking/calling with hoping to trap. I realise reads on opponents reactions to calls and aggression preflop will come into this but generally speaking??

Cheers.