Generally, you go by what they call "cash basis"--you count the money in the year you get it. In your case, since you haven't yet gotten paid for the work, you don't count it as 2014 income. You'll count it in 2015. The same would apply to expenses such as business expenses or medical bills--you count them in the year you pay them.
This applies to individuals with side jobs and straightforward returns. Businesses are different, as are complex returns.