When to make a continuation bet and how much in Texas Hold'em?
I would say that you want to make a continuation bet a large percentage of the time if you have signalled you are in the lead and no opponent has signalled otherwise.  What this "large percentage" is probably depends on what you are comfortable with and how aggressive you are, but I would estimate it would vary from 50% to 100%.  Continuation betting on every single flop will make you predictable.  You should also factor in how tight the table is - placing the bet more at a tight table and less at a loose table or against loose opponents.

Don't vary your bet based on whether or not you hit the flop.  This gives a signal to your opponents which will eventually allow good opponents to tell whether you hit the flop. Vary your bet based only on the texture of the flop - as you mentioned, a flop with more draw possibilities rates a larger bet, although some flops may be so dangerous that you may not make a continuation bet at all (i.e. Jh, Qh, Kh) if you did not catch a piece of them.