if you are doing a sharp turn without pedalling also have your inside legs bent, this allows for better control of centre of gravity with you other leg. also as previously mentioned look where you are going and make sure you have enough speed if you are doing the turn. best thing to do is get some witches hats and go to an empty car park near you and put the hats out or any sort of marker and just practise.

i cannot give any advice on standing tho, that is just a natural thing to me, although my coach is getting me to stand and sprint while hands in the drops, that is a weird feelings, and man my butt was sore becuase of the way he told us to ride. maybe you could get speed up on flat ground and stand up without pedalling just to get a feeling on it.