I normally don’t link articles I see on reddit on the theory that you’ll already have seen them, but this one was too good to pass up. New Artisans has a superb article on iptable tricks to defend against common attacks, and even provides some hands-on examples of what they’re defending against. If you administer your own server, I strongly recommend taking a look.
(Note: try the “attacks” they show only within your own LAN. A competent ISP may notice what you’re doing and shut you down if you try some of the simpler things they talk about. Besides, it’s just plain not nice.)