The easiest, and potentially the cheapest, way is booking semi-flexible (Q class) tickets with TK. They are usually around $1000 for a round-trip between Europe and the US. 3 round-trip IST-LAX flights already get you elite membership from 0. Don't book discount economy with LH as you will get 0 miles. United is slightly better, you usually get 50 or 75%. However, the best redemption value is still flying TK in Q class.