Don't have to declare it to the USA? It's not that easy.
A few general things. US citizens and permanent residents (Green Cards) are liable for taxes on any income obtained anywhere in the world. You earn $1 US dollar, you are liable for the taxes on that $1 US dollar.
Now, with the deductions and exemptions that are available to an average US tax filer, there is generally no taxes due for income under a certain amount. There is a very big exemption available to SOME US expats (note: not all) if they meet a specific test for living overseas or have been out of the country for 12 months or so. You also get exemptions or credits for certain taxes you pay to other governments. Please read the IRS guides and rules. This leads to the general principle "$0 in taxes owed, no need to file a tax return".
However, every accountant I've ever met usually advises their clients, and especially those living overseas to file every year regardless of whether you owe anything or not. Get it on paper that you have no US tax liability for each year. It's good to have paper proof that you did file, not to find yourself in a situation in say three years where you need to produce your tax documents and say "I didn't have enough income" and have the person be like, "ok....yeah....sure..." Better to just get it on paper now.
note: You will also need to declare any foreign bank accounts you hold if you have (at any one point) over $10,000 US in that account. You also need to declare any ownership interest in foreign companies if your percentage of control is greater than a certain percent.
Please don't take this as definitive advice: review the IRS publication for expats:
http://www.irs.gov/pub/irs-pdf/p54.pdf