Giving your prospect a deadline for ordering, particularly when that deadline is a date and not simply a period of days ("Order within the next 30 days"), will outpull mailings with no deadline almost every time.

But you need to be cautious about deadlines.

If you are making a time-limited offer, give a reason. And make it a good reason. Otherwise your readers may be skeptical. Your time-limited offer needs to be plausible. And it shouldn't make you look greedy. A good example would be a line like this:

We need to receive your order before 15 April because our prices are going up by 20 percent after that.

Also be prepared to see inquiries die on your deadline. If your prospects and customers take your offer seriously, as they should, do not plan on receiving any more sales after your deadline arrives. If you extend your deadline once, and then extend it again, you'll create the very inertia that your deadline is trying to overcome.

Plan your time-limited offers carefully. If your cut-off date is too soon, your offer may arrive on or after the deadline. I recommend that you mail first class for this reason.

And if your deadline is too far off, you'll encourage procrastination.