Don't keep all your bitcoins in one wallet.
Don't develop a carefree attitude towards wallet security. Handle bitcoins with the same amount of diligence as you handle physical cash.
Don't use a password for wallet encryption that you use elsewhere on the web.
Don't use weak passwords. If a password is easy to remember without rehearsal, it is probably weak.
Don't forget your passwords. If you write them down on a piece of paper, keep that piece of paper in safe storage (safety box, etc.).
Don't run bitcoin wallet software on an untrusted or badly maintained machine.
Don't run software that has been copied illegally (eg, "warez")
Don't send bitcoins to a wallet that hasn't been backed up beforehand.
Don't send unencrypted copies of your wallets over the Internet.
Don't store unencrypted copies of your wallets on online backup/storage services.
Don't run or install programs from questionable sources on the system where you are running bitcoin wallet software.
Don't insert any devices of doubtful origin.
Don't store unencrypted copies of your wallets on a device that you insert into machines of friends, at work, or while traveling.
Do wipe your HD and perform a clean OS install if you believe your computer has been compromised.
Do keep at least one offline savings wallet.
Do backup your spending wallet regularly.
Do keep dated versions of each backed up wallet.
Do encrypt all wallets as soon as they leave your system.
Do keep encrypted backups of all your wallets in at least 3 physically separate locations.
Do use strong, unique passwords to encrypt your wallets.
Do update your operating system with the latest security patches.
Do invest time and money in order to secure any large number of bitcoins you may have, just like you would with other valuables. If you lack the technical expertise, ask or pay somebody else to do it for you.