The HTC Trade Up program can get you up to $300 CAD off a new HTC One, provided you live in Canada and act fast.
I'm saying you should act fast because, while easy to take advantage of, the offer (which is not a first, HTC did a similar promo not long ago) is only available until May 27. Here's how the program, called “One Phone, One Weekend, Premium Upgrade”, works.
First, go out and buy your new HTC One, then go to the program's
website (link below) and use the form to enter data about your old phone
(you'll see that it's pretty simple). When asked to enter a promo code,
input HTC100. You'll get the trade up value for your old phone, and the
next step will be to mail in your old device by June 27, along with the
proof of purchase of the HTC One.
You'll get a prepaid credit card with the value of your old phone
(which can be between $100 and $300 CAD, but not less than $100).