Varying positions is one of the best ways, as well as making slight adjustments within each position.
The Missionary position can seem somewhat of a cop out but if your penis curves upwards this will stimulate the females orgasm zone for reasons pointed out in the previous statement.
Golden Arch Emily Schiff-Slater Do It: Your partner sits with their legs straight and you sit on top of them with bent knees on top of their thighs, and you both lean back.
Make It Hotter: Stimulate your clitoris with one hand, or ask your partner to do the finger work for you.