Not to put a damper on this but I drink heaps of water (heaps!), eat an orange a day, plus have orange juice for breakfast and fruit on my cereal. I have plenty of fibre and I pooh correctly (was taught in an early pregnancy class) - and I still got a hemorrhoid (it is nearly gone now).

I don't know that there is a fail-safe way to avoid them.

BTW - I actually think I got mine from going to much rather than straining. I used to go once every day or two and then since being pregnant can go two or three times a day.