By indicating that an offer is due to run out or an item is going to run out of stock, you're more likely to inspire action. People won't want to miss out on something they think will soon be gone, so they'll be driven to click. Try running an ad referencing a time-limited offer or a "last chance to get X product" to drive up click through rate.
“Only 15 limited edition razors left!”
“Last chance to get the designer-branded jacket line!”
“Only 10 spots left in our email marketing webinar!”