How to set a discount for all products?

I've setup a website and try to make SEO optimization, now I have 100 IPs everyday, but few really buy items. So, I want to set a discount on all products at a promotion period, after the promotion period, all price is set back to original. How to do that?
I really don't like change price manually, I prefer use discount to affect prices.:-)
Can anyone help me? Thanks in advance.

https://www.580mall.com

SaleMaker can do that easily. Just put in your start and end sale dates, percentage or amount to reduce each price by, and select "entire catalog" and it should work like magic. ;)

Thank you very much! It's very easy using catalog->salemaker.:-)

Yeah, sales & specials are two really strong ZC features IMNSHO. I have ongoing discounts (on my Quilt Patis, which are marked down from suggested retail) and run occasional sales, and ZC has made that a breeze (once someone told me that if I select categories for sales/specials to apply to, I have to make sure those are the main categories for those products).