I haven't used one for a while, but I've tried a couple and had the most success with the 'spend X amount, get X percentage off the total' promo, but I've also tried the buy one get one free with almost no one taking that up.
I think you'll find that promotions aren't really conducive to increasing sales volume, but more for increasing spendage from the people who do buy (most of them are designed around giving the buyer bonuses or discounts if they spend more, or in the case of BOGOF, buy more). That can act as an incentive to get people to buy, but no one sees the deals unless they're already looking at items with the promo, so it's almost impossible to increase traffic with them (even when you do a straight discount off the item price, this isn't indicated in search results, people don't know the listed price is discounted, so there's no immediate sense of "I can get it cheaper if I buy it before the promo ends"). If you have a social media presence, you can advertise the promo and increase awareness or interest that way.
I personally believe the reason it was successfull is because it was in line with how people buy from me already - i.e. I sell the kind of stuff that people tend to make orders with multiple items, giving them a discount when they reached a certain spend acted more like a deal-sweetener than a deal maker, but was quite popular all 3 times I used it (actually, every time I ran a new one, the total qualifying spend increased by about 20%).
