I can blow squares - and triangles [sometimes, most of the time the triangles dont work] but smoke shapes with staight edges only laste a couple of secs tops, then the smoke goes wavy and it's gone.

Smoke rings are easy though. Just kind make a circle with your mouth, as though your going to whistle, but slightly bigger and push a bit of smoke out fast - that should give you a smoke ring. I'm not very good at describing stuff.