1)put a "bubble pack" solar cover on the pool when not using it but only when you want it warmer. This is the cheapest thing you can do.
2) run a black plastic hose or "solarroll" (flat black plastic with small tubes ) all over your house roof and tied in to your pool filter in a way that you can shut off some vales and take it out of the system (because of too much heat or to drain) It needs to be a closed system so the pump has no additional work to do. This will have the added benifit of shading and cooling your roof.
There is a tradeoff of absolute eficiency and cost efficiency between colectors in boxes (less surface area) and just pipes.
3)don't even think of using copper!
Toofuzzy
Take the road less traveled. It will make all the difference.