Some of these are repeats... but they deserve being repeated!

• Do encrypt (passphrase protect) your wallet. Don't use a simple passphrase. Don't use a passphrase you use elsewhere. Encryption helps, but understand that it doesn't prevent all types of theft.
• Do backup your wallet. Different wallet software work in different ways: some wallets you only need to backup once, others you need to backup after every new receiving address is created, and others are somewhere in-between. Understand how this works for the wallet software you've chosen, and be vigilant with your backups!
• Don't start off owning more BTC than you can afford to lose/have stolen. If you find yourself getting close to your personal limit, search for "cold storage" and find out how to use it. Don't put this off until it's too late.
• Do choose one of the wallet options from the official choose-your-wallet page here: https://bitcoin.org/en/choose-your-wallet. Not all online wallets are bad, however Do heed the warnings on this page regarding some of the less-safe online wallets. Choosing an online wallet does not exempt you from backing up your wallet.
• Don't download programs from the Internet. This rule is difficult to follow 100%; just be aware that each new program you download and run could be the one that steals your BTC.

Regarding that last rule, here are some guidelines that can help you decide how (un)safe a download is. Note that these are not hard-and-fast rules!

• Do research a program before downloading it. Are there a lot of websites discussing it if you search for it? Does it have a Wikipedia page that wasn't created yesterday? If either answer is No, Do be more suspicious of it.
• Software that is open source, especially if the answers above were No, is generally more trustworthy, but Don't assume that something is safe simply because it's authors claim it's open source!
• Software that is written in a popular scripting language such as Python, Ruby, Perl, JavaScript, etc. is generally more trustworthy, but Don't assume that something is safe simply because it's a script unless you can read and fully understand the entire script! Never download and run something claiming to be a script if it's actually an EXE file.
• Do look at the number of pages in a thread before downloading any software referenced on the first post of the thread. More pages means a smaller chance of the software being malware (but there's always still a chance it could be malware!). Do avoid software with only a few pages worth of posts - let someone else be the guinea pig!
• Do look at the Activity count and the Date Registered of users when trying to decide if they're trustworthy, and be more suspicious of Newbies, but Don't automatically assume that a Hero Member is 100% trustworthy.
• Do err on the side of caution/paranoia. It only takes one piece of malware to steal all of your BTC even if you've encrypted your wallet and even if you're using two-factor authentication.
• If you've decided to risk downloading something, Do take the time to find the single official download link or website.

Sorry if I sound scary... most people around here are the honest type, but there absolutely are scammers around here too. Use common sense, and have fun!