Take a deep breath. They are not going to yell at you for flubbing a line, considering you had no prior knowledge of the material.
Ask for a brief moment to read through the short script (usually just 1 to 2 pages).
Do not, however, try to memorize lines or derive character information from such a brief glimpse.
Do not be afraid to glance at the script before speaking a line.
If acting with someone else, though, be sure to listen when they speak as opposed to sneaking peaks at your next lines. Listening is very important.
Don't flail the script around, drawing attention to it. Body language should be minimal here.
Get the stigma of cold readings out of your head. Pretend you are doing exactly what you normally do, in order to help stay in character.