Most of the time by project. After doing this for so long, I have a pretty good grasp on how long something is going to take me so I will quote you a project cost and I will stick to it. The only time I do not stick to it is if YOU don’t stick to it. For example: Say you want a postcard. I design this postcard and I include 2 rounds of revisions. After the 2nd round, you decide you want to completely change the design or you want to change the size, the content etc. Now we are going by the hour, which is $70/hr.
Other examples of when I would charge by the hour is if you are an established client and you are want to make some text updates to your website.