Creating discount code to do 25% off of only one item

I'm having a little trouble figuring out how to do a discount code. Let me simplify first by saying that all of my products are the same price ($45). I want to do a discount of 25% off any one item and not apply the code to a second or third item added to the cart. I set up something kluge that works (taking 25% off orders $45 and under, taking 13.5% off orders $90 and under, etc), but it's certainly not ideal, and the logic takes a little bit to apply when entering the code. Any thoughts?