The slogan "Buy One Get One FREE" is one of the most effective promotional invented. Studies show that it beats "50% off" even though both offer the same. This is because, marketing-wise, FREE is very powerful.
Here are some tips for using it:
1) It is better for cheap items where customers are prepared to stock up.
2) You need to pick items with good margins; after all, you are giving away half the money. Some people would use something like this
Buy two and get another free.
Here at least, is only a third off.
3) Works better for flash sales. Try running it during a festive season, for example, as a Father's Day special.
4) Many products customers cannot use two of the same product. For example, two jackets for the price of one might be a problem, but two pairs of socks for the price of one should be okay. In which case, for jackets, consider something like
Buy a jacket and get One from Category B for Free.
This Category B has proven to be a great way of selling stock you no longer want.
Setting it up is easy. All you need to do is follow the red arrows in the image below.