What I'd do is I'd probably offer new customers a whopping 75% off on the day. And then continue the promotion for a further week but at a rate of 10%-25% off. And at the end of the week, revert to standard prices.
Then post a quick ad to let people know about your deal.
And see how much attention you get.
People will always act fast if there's limited time. 🙂
Alternatively if you're an established company. And are more interested in promoting your existing customers rather than new ones.
You could offer them more hard drive space, etc. at no extra cost.
Or a discount off the more expensive packages if they upgrade in the promotional period. And an extra special discount if they upgrade on the day. 😉
And for those that don't wish to upgrade, you could offer them a discount off next months bill. Say for customers who've been with you for 6 months or more.