Frankly it depends on tax and how you are declaring it.
Disclaimer: US custom/law may vary from UK custom.
Top:
Invoice (number and any job/estimate reference/title so they can tie it in into their own paperwork)
Name, address, contact details
Hourly/piece work rate, sub-total.
Expenses etc.
Total
Terms and conditions (i.e. to be paid from 30 or 60 days from invoice etc.)
Not much but it's a start.