Calling an All-In Heads Up

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.

i usually do it ace rag cause most people are willing to ball kj kq or even krag and i just flip the coin...probably a poor philosophy id wait for everyone elses comments

Sort of a tough question, because HU play is very dependent on reads on your opponent. If opp has been "normal", at a minimum I'd call AI with those big blinds with any ace, any pocket pair, say KT+ and maybe QJ. If opp has been very aggressive, I'd open up that range to include more Kx hands are say Q8+. If opp has been folding a lot, I'd toss the lower aces and probably the Qx hands.

I can't find them right now, but there are tables showing how various poker hands match up against other hands, and high card power usually rules in heads up AI confrontations. (EDIT: here's the link - http://www.jazbo.com/poker/huholdem.html)

In terms of hands to raise when you're on the button/SB, again it depends on your opp. I tend to raise like a maniac against tight players who fold too much but against loose players I dump the worst hands (crap like 42o, 63o, etc). With marginal hands like J6o and even stacks I might just complete the SB if opp will let me see a free flop by checking the BB.

It's sort of difficult to give general advice on HU play, the best way to get advice is to post a series of HU hands with your reads on your opp and we'll take a look.