There's no way to really toughen them up beforehand. In fact back when they used to tell women to do that, there were more issues with latch and whatnot. After the initial week or two of breastfeeding, it shouldn't hurt your nipples. It'll hurt right at first while you get used to the sensation but if it doesn't fade, get ahold of a lactation consultant.
Honestly I don't think there really is a way to get them prepared.
I don't think theres really a way to get them ready. They are made to be used this way. Discomfort and problems happen due to bad latches mostly. I'm not a doc, this is just my uneducated guess, so maybe talkin to your doc or if your mom breastfed you see what she did.