Chris Rand says: “Form design is an area where most web designers really could do with some training, if the forms I come across on a day-to-day basis are anything to go by.”
So, even if you are trusting a web designer, better to have a best practices cheat sheet to hold them accountable to. That’s exactly what Chris links to, posted at Smashing Magazine.
Speaking of usability, the download has many different file formats in a zip file. If you just want the PDF (print to 11×17 for legibility), here it is.
(Secretly wishes the zip file had CSS code for the examples.)