If you want to register new domains in March 2009 from Namecheap, here is the coupon code you should use: SPRINGSUN. With this code, you will be able to register domains for as little as $8.81 (instead of the regular 9.69)! Namecheap occasionally releases renewal coupon codes through their newsletter. You should subscribe to their newsletter from within your account.
If you have more than 50 domains with them, make sure you contact their support and submit a request for a domain renewal coupon code. The domain renewal coupon is available only to their high-volume customers. It would not only save you money on domain renewals but also Whoisguard renewals, just in case you use it!