Isn't that a Nike slogan from a couple of years ago? Don't know about a movie....

Starsky and Hutch

More than one... - "Jesus Christ, woman, must everything be a full-scale debate with you? Just do it! Medicine Man - 1992" - "(standing next to the gate of a wight pickett fence) 1) It's locked. From the inside! 2) Were gona die, were all gona die! 3) Get a hold of yourself. Blow it! 1) I can't, it's not our property. 3) Just do it! Hot Shots! Part Deux - 1993" - "1) I know the man who killed me. 2) He knows who killed him. 3) He's name's Willy Lopez. I know where he lives. 4) He says his name is Willy Lopez and he knows where he lives. 5) Write it down. 6) He wants you to write it down. 7) YOU write it down. 8) I ain't no damn secretary! 9) Just do it! Argh! Ghost - 1990" - "1) Kick him in the nards, kick him in the nards! 2) Wolfman doesn't have nards! 1) Just DO IT! (*kick*) 2) Wolfman has nards! Monster Squad, The - 1987 " - "David: Tell her to pack up and leave town. Marty: Why? What happened? David: JUST DO IT! Independence Day - 1996" - "1)Ask her if she would go out with anybody from class. 2)No! 1)JUST DO IT!!! Billy Madison - 1995" - "Sara: Here you go. Write your name and number down.<BR>Jonathan: On this $5 bill?<BR>Sara: Yeah, just do it.<BR>Jonathan: You are a strange and interesting woman. Serendipity - 2001" - "1. Don't shut YOUR eyes! 2. Sorry. I just got a little nervous. 1. YOU'RE nervous? An eleven-year-old is cutting my hair. 2. Hey, you sounded just like me. 1. Well, I'm supposed to, aren't I? Go on, just do it. Parent Trap, The - 1998" Source and further information: http://www.moviequotes.com/searchquotes.cgi?SrchQuote=just+do+it

it's in My girl also