Hoare Logic Loop Invariant

I'm looking at Hoare Logic and I'm having problems understanding the method of finding the loop invariant.

Can someone explain the method used to calculate the loop invariant?

And what should a loop invariant should contain to be a "useful" one?

I'm only dealing with simple examples, finding invariants and proving partial and complete correction in examples like:

{ i ≥ 0 } while i > 0 do i := i−1 { i = 0 }