Unwrapping is a skill in and of itself. There really is no right or wrong. You have to judge that yourself.

I made a [fairly extensive tutorial][1] on it, but here's a quick overview:

1. Figure out where to flow of the mesh and plan where to place the seams. Wherever you place a seam, Blender will "cut" the 3D model and try to flatten it. You want to make sure first and foremost that it will be able to split apart into even planes, otherwise you will get major warping issues.
2. Open up a UV editor and start placing the seams (<kbd>Ctrl</kbd><kbd>E</kbd> > Mark Seam).
3. UV unwrap the model (<kbd>U</kbd> > Unwrap) and make sure it unwrapped the way you expected.
4. Tweak the seams until you're happy with the result.

After you unwrap a few meshes you'll get the hang of it, and you'll be able to unwrap quickly and cleanly.

  [1]: https://www.youtube.com/watch?v=e9p9dYcvIzc&list=TL60v8BSV0JXXyGJ5pGk-vsEGJwr8p7PB4