Requirements

Anyone know a good requirements analysis methodology that doesn't cost the earth and take forever? I have some requirements that I want to formalise, validate and get signed off quickly.

The way I see it, there's a triangle - excuse the ASCII art. Must work out how to get piccy's in this thing.

Problem
/ \
/ \
Requirement -- Solution

If any arm of the triangle breaks, the whole thing falls down. So I think I want a way to validate the links.