The sloping curves of a skatepark are sometimes called "flow". Skateboarding over flow, and up and down slopes and ramps is a little tricky. The first key is, always keep your weight on your front foot. So, when riding over a big bump, down a hill (be careful there are no cars!), down your driveway, or through a skatepark, keep your weight on that front foot. That doesn't mean to tense it all up - you need to relax, too!
There is one trick to this key - when you ride up a ramp or slope, pause, and then ride back down fakie, your front foot just changed. Do you understand? Your front foot isn't always your right or left foot, it's the foot that is facing the direction you are going! Do, in when riding up a ramp or hill and coming down fakie, you'll want to transfer your weight from one foot to the other right at the top. It might take some practice!
The second key is bend your knees. Keep your knees bent, and as loose as you can. This will help your body to adsorb the shock and impact of bumps and changes. As a huge rule in skateboarding, the more relaxed and bent your knees are, the better you will skate. And don't hunch your shoulders too much, either. Try to keep your shoulders back a little, and relax. If you hunch a little, that's ok. The main thing is to be relaxed.