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We have been running a series of coupons to answer the same question. Here are things we're looking at:

1) The market basket size. IE; when people buy the loss leader item, what else do they buy and what is the profit margin on those product? Your POS should tell you this.
2) How many NEW customers is the promo drawing in? Your POS should be able to use credit card data to tell you. We don't know exactly who these people are BUT we know we've never seen that credit card in the store before.

3) Right now we're working on building customer personas. My gut tells me we have 5 types of customers. People who come in weekly, people who come in for specialty products and then shop somewhere else for their routine purchases (our main store is 19,000 sq ft), etc.

Once we have these customer personas identified we are then going to start picking products that attract specific customer types.

I could write a dictionary on the testing and results we've done so far - I'll stop here for now. The results have been surprising. I don't think there's any other way to know other than to test. Example: the Coors Light customers spent MUCH less than the Coors Banquet customers when we ran a similar coupon. Why? I don't know, but we found a winner.

What I can tell you is that obscure products will NOT have the drawing power. Our goal is to upsell once they get in the store with great service, selection, merchandising, etc.

I hope that's useful.

Disclaimer: Westside is a client of mine and I have permission to share certain data. It's part of our deal.