I used to slice the ball as well (I think most people do in the beginning and some people just decide it's easier to play with it than fight it...but dont give in!). I tried fixing 1000 things, like my grip, backswing, timing, followthrough...and the truth is, they all help and they all can hurt at the same time. By fixing my grip (opening your grip hand...your right for a lefthanded golfer) I changed my slice into an aweful hook. By shortening my backswing, I made much straighter shots but the club was defaced at impact and was a low line-drive and not way a 10.5 degree driver should hit.
I am not trying to promote one thing over another because truth be told, everything makes a difference, but I went to the range after watching this video and hit 8 for 10 dead straight and perfect trajectory. The video is on youtube.com and features a lesson from Shawn Clement, a PGA pro and teacher...who also has his own site with ton's of videos to look at to help you. I am linking in the source section the specific video that may help you the most in my opinion. Pay particular attention to the part where he talks about "swinging above the ball before your backswing to get the pendulum effect going".
By placing the driver above the ball, swinging it loosely forward and then directly into your backswing, you have already created the proper swinging plane. All you have to do then is let your arms fall forward and strike the ball. The video shows in more detail what I am refering to and I promise that this excercise works. I tip my right knee inward just before the backswing to force my body to rotate, not pull the club back. This also helps keep the plane in the right line. Lastly, something not mentioned in the video is you HAVE to followthrough to your target. You want your belt-buckle to be facing the target and keep your swing all the over your shoulder. When practicing, I tend to "throw" my hands toward the target during and after contact. This keeps the swing straight and keeps the followthrough high, not sideways. I hope this helps and good luck!