I didn't need any special preparation before starting on tamoxifen, although the doctors did tell me to wait for a couple of weeks after finishing radiation. I'm not sure why, but I think the doctors thought one thing at a time is enough to deal with. I finished radiation on the 14th March '12 and started the tamoxifen on the 1st April, I thought April fools day was a good day to start !!!
I have so far been lucky that I didn't have any bad side effects with the tamoxifen although I did have to stop it for a month around the time I had the breast reconstruction surgery (I think it might have a blood clotting risk).
Good luck and stay well
Penny