I know what you mean...I was having the same thoughts a little while ago. Everything seems to have tightened, which is great if you're not about to push a baby through there I find, personally, that DTD....backwards.........helps to stretch out that perenial area, plus I find that I can make myself far more comfortable during the process than if I'm on my back. If you're finding you're tensing yourself up, have a nice warm bath first, and get DH to concentrate on you for a bit before hand, nice massage and some foreplay, and it should help to get your natural lubrication happening as well and relax those muscles.

Good luck x and happy stretching