Back up the wallet.dat and you only need to do it once (unless you create more than 100 receiving addresses).
This is not true. You need to back up regularly. Bitcoin-Qt uses a new address (that it keeps hidden from you in the wallet.dat file) every time you SEND a transaction. It also uses a new address every time you click the "New Address" button. When you've use up 100 address (including the hidden ones from transactions you create to send bitcoins), your backup will no longer be any good.