As for prep 'down there', unfortunately as far as I am aware if you're lucky and have a good labour you may not tear but nothing can prevent it from happening.
My dr has told me pelvic floor exercises are very important to help with a natural birth.
As for being 'stretched', I haven't experienced this. Our bodies have an amazing way of healing themselves and the affects of birth do not necessarily cause a permanent change down there besides maybe a little scaring. The main effects I found were with my tummy muscles for obvious reasons but that can always be worked on when you're up for it again.