How to improve front brake functionality?
It sounds like the cable's slipping after you adjust it.  

Precisely how you adjust the cable depends on the brake design, and normally you would not shorten the straddle cable, since it needs to be a certain length for optimum operation.  However, with many newer bikes the main cable makes up half the straddle cable, with a spacer to maintain the proper straddle length.

Sheldon Brown has an article on cantilever adjustment, though it's a little short on actual adjustment operations.  Park Tool's article, on the other hand, almost goes into too much detail.