Hi Jan
Can i just add some things to what Don has already said.
NEVER stick your rear end out while dancing.
Here's a little exercise to show what's meant by having this nice backward poise which you see so often in the good dancers, and yet having the weight forward.
Stand in a good upright position, and then take your weight onto your toes by standing on your toes. By being on your toes, your weight must be forward over the standing foot.
Now, staying on your toes, stretch your upper body up and slightly back in the 'classic' lady's dancing position (don't overdo it). Now you're poised back, but your weight is still forward, since you're balanced on your toes. You will notice that your hips move forward (into your partner, if he's there) this is inevitable because of the laws of physics, which you will never be able to defy!! But it's a good thing, because it keeps good contact with your partner. As Don says, you must keep this poise and balance as you move, but practise it statically first.
If you're concerned about the flexibility of your upper body, you may find the 'cobra' exercise of great benefit for making your upper back very flexible. Do it 3 times a week, and after a couple of months, you will notice a great improvement.
Good luck