I would recommend monthly. Especially if you have lots of "friends". I quoted friends because some people aren't always who they say they are. That is a different story. Google up Social Networking Engineering.
Tip 1: I would recommend keeping it totally different. I've heard stories of people loosing their MySpace accounts and a whole mess of trouble came after.
Tip 2: Avoid logging into MySpace on public networks, that you don't totally know. You don't know who is sniffing packets in the network. MySpace, is a popular one for packet sniffing. Packet Sniffing, is where a hacker will run software and intercept all data transferred. For example, login data, google searches, etc.
Tip 3: Use unique password. Don't share it with any other account. Most hackers will assume you will have the same password for all. If they can guess one password for one account, they will take over all accounts.
Tip 4: Don't share it. Even with your boyfriend or best friend. You never know if they wont betray you just once.
Tip 5: Use capital letters, numbers, and possible other characters. Avoid capital letters and numbers in beginning, middle, and end. That makes the strong password.