As Svar said, it is very hard to buck someone off if they know what they are doing.
Your hips are naturally strong, very strong. If you want to know how strong they are, grab a Swiss (yoga) ball and hold it to the front of your body then just walk at a normal speed into the wall. You'll be shocked at how much energy is stored just in your walking.
IMO, most people don't have a problem with hip strength, their problem is not knowing how to properly move using their core. I've been doing grappling arts for over a decade now and it wasn't until I started doing aikido and yoga that I started learning how to move with my hips. It is something that I cannot teach you over the net, but I can give you a pointer.
When you move though your hips make sure that when you turn or rotate, you rotate your whole body. Many people just rotate with their upper body and their core is more or less stationary (I see this a lot in MMA), which in turn requires a lot more strength to pull off. Do what Svar suggested and make sure one of his sides is well trapped. (i.e. so he cannot balance himself easily once you break his balance) then try to escape using/turning your body as a whole, not just bucking your hips.
One other tip I can give you for practice is find a partner and just working on moving their (more or less) dead weight. Exaggerate your movements. That is, try moving him with a bad movement (i.e. just your upper body with stationary core), then try to correct yourself by focusing on using and turning your whole body with the core. This can be applied to take downs as well as escapes.
An exercise you can do, is the first part of a Turkish get-up. I do these with my kettlebell, but you can also use a dumbbell too. Lie flat on your back, extend one arm (with the weight) over your chest. Just like you did a one arm bench press. Now explode up into a partial seated position and stabilize yourself with the free hand. You can do versions with lighter weight where you do not bend your knees or heavier sets where you bend your knees to take pressure off your back. Be careful not to throw your shoulder out. Doing this exercise and being mindful will help you determine which position each part of your body needs to be in and move in order to have an optimal movement. If you cannot decipher what I mean in the description, here is a video. Turkish get up
My description is just the first part, but you can do the whole exercise if you want.