There is a product that you can buy that I personally have had good luck with in acelerating fin regrowth. It's called mela-fix and can be bough at most lfs, as well as online. It is based on tea tree extracts, and it is mostly an infection inhibitor, so fins can regrow without also having to work around any kind of infections that might be workin on the wound.
If is about 5 dollars for the smallest size, and I had no problems putting it right into a tank with plenty of LR. It says it is safe for all inverts, but I don't know if I would trust that if I had a tank full of delicate clams and what not.
If you don't want to use a product like this, just make sure you have good water quality, and that you do very frequent water changes if you arent running a big skimmer to keep the chances of infection at a minimum.
GL