If your iTunes account is associated with an Australian address, then the options for what you can buy online will be different (and generally more limited) than those available to US iTunes users. Set up your own US iTunes account and you'll have access to a much wider range of content to purchase.
Presuming you're not worried by providing a not-exactly-accurate US address, operating a US-based iTunes account isn't difficult. The only major restriction is that you'll only be able to top up your balance with US iTunes cards. While these are regularly sold via eBay and through various online stores, you'll have little recourse if you get sold a dodgy code -- grabbing some on a US trip or getting an American friend to buy them for you is a safer option.
The steps below tell you how to set up a US iTunes account. These instructions are based on iTunes 10 for Windows, though the basic process should be very similar on a Mac.
Open iTunes on your PC. (If you haven't already installed iTunes, you can download it from the Apple site).
If you are already signed into your existing Australian iTunes account, make sure you sign out. Click on the signed-in address in the top-right corner, then click on the 'Sign out' button in the dialogue that appears.
iTunes generally copes quite well with content acquired via different accounts -- the biggest nuisance is that you'll have to sign in and out of each account to view the relevant content or use the relevant apps. If you want to keep the process completely separated, you can set up a second user account on your PC and use that for accessing your US iTunes installation, but that's probably overkill.
Click on the iTunes store link on the left-hand side of the screen. Scroll to the bottom, and click on the small Australian flag in the bottom-right corner. A list of countries will appear. Click on 'United States'. (You can use a similar approach to create an account in any iTunes store, but we're concentrating on the US here.)
Click on the App store button near the middle top of the screen. Scroll down to the 'Free apps' list on the right, and click on the 'See all' link.
We're going to install a free app in order to trigger the account creation process. It doesn't matter which app you choose (or even if you plan to use it) -- just click on the 'Free' button to download one that appeals. Click on the 'Create new account' button in the pop-up that appears, then click 'Continue' on the splash screen that follows. Tick your agreement to the terms and conditions, then click 'Continue'.
The next screen will ask you to set up your Apple ID. Don't use the same address you already have for an Australian account. You can always use a disposable Gmail address if you don't want to create an additional email for this purpose. You also need to include at least one upper-case letter, though the software doesn't initially make that clear. Make sure you deselect the email newsletters (unless you really want them), then click Continue.
The next screen asks for a payment method. Select 'None' for the time being.
You also need to provide a US address to complete registration. One common smart-aleck response is to use Apple's own address, but that's the kind of trick it might easily check for. I'm not going to provide a specific address to use -- that would seem a tad unfair -- but I'll suggest that any US-based hotel address will work nicely. For the phone number, you can just use 000 0000000 -- iTunes demands a number but doesn't verify its accuracy.
You'll be sent a confirmation email. Click on the 'Verify now' link, sign in with the password you have just created, and your US iTunes account will be set up. (To add credit via a US iTunes gift card, click on your account name and select 'Edit payment information'.)
Got your own strategies for buying US iTunes credit and content? Share them in the comments.
Lifehacker 101 is a weekly feature covering fundamental techniques that Lifehacker constantly refers to, explaining them step-by-step. Hey, we were all newbies once, right?