The best way to put weight on your heels is to jump without stirrups! Do it several times and you'll see that your position will improve very fast.
This will force you to be really sat on your saddle and close to your horse (otherwise you're down ;) ).
Don't forget to open your knee, this is usually why you can't put weight on your heels.
If you have difficulties with following your horse, try to ride while closing your eyes and just feel his movements, it will help you be with him and have better actions to lead him.
You can also try to ride with a dressage saddle sometimes to really "feel" the right position.