I had issues with the rising trot up until recently. I have learned that the more forward (faster) my horse trots, the more fluid the action of posting is. When he trots slow, I have to make more of an effort to rise up out of the seat. I barely have to try when he is forward, I really do feel like I am naturally being lifted out of the seat. Also, pay attention to your legs when you are posting. Your lower leg should be gripping your horse like a big beach ball, the rising should come from your upper leg. Think of it like you are standing up from a chair, and bring your hips forward towards your hands.
It does take practice and you'll have to build up your strength and stamina, but you'll get there. I am a plus size rider, so if I can do it, you can do it!