Middle part help

So I always middle part my hair, my go to method is to shampoo and condition my hair and when I get out of the shower, I comb all my hair straight down and then part it. Sometimes, my left side doesnt want to part correctly and it just goes downward and all poofy and stuff. I want it to look like the right side but it just starts to do this when it gets long. How do I get the perfect middle part? Please help