Pin the netting to the back of the fence posts - and the single rail to the front - helps stops ponies getting to close depending on size - but in all honesty if a pony gets that close to a rail its broken anyway

If the field is used for lambing - its just a case of checking them on a regular basis. For sheep in general - if they are sticking heads through fences, maybe you need to put them in a field with more grass in (although they do regulary get stuck anyway I spose

The only field I have post and railed with two rails and sheep netting is the campsite - and thats just cos it looks good for campers in summer and the only field that it was financially feasable. All the other fields are stock fence and barbed.
I'm more worried about electric fences than stock fence - at least on your checks you can pull a sheep out after a few hours and its ok - electric netting they are dead in 10 minutes.