What should I do to prepare for an eye exam?

Not much. Bring your eyeglasses and contact lens parameters if you wear them. Know what your eye history is and what medications you are currently on. Be prepared to have the eyes dilated which can induce glare in the sun and sometimes makes driving difficult.
Dilation. Be ready to be dilated (some like someone else to drive since dilation blurs near vision occasionally and can cause glare temporarily). Bring any old eye records and glasses, current eye drops. Free up your schedule since a new visit might take awhile.