You can get promo codes from almost any DreamHost customer (just take a look at the post signatures around here ). Different promo codes do different things. Some promo codes offer plan discounts (A hosting plan that was signed up using a promo plan will automatically renew at regular price unless you set the account to not Auto Renew), other promo codes offer lifetime benefits, or a combination of both. You can only use one promo code during sign up. Promo codes are usable for new sign ups only. Not all codes offer equal value, especially depending on the situation of the new customer, so choose wisely. For more details on promo codes, see:
I should note that I have a link to my review of DH and my promo code in my signature.