you need to use key frames. The way you do this is by selecting the video you want to edit in the timeline and then the part you wish your video to start slowing down/speeding up, once you have that selected you go to effect controls and in Time Remapping you click the arrow to apen the drop down menu. Once you did this it will show (on premiere cs3) the Velocity / second, you select the velocity you want your video to start and you click the dot next to the speed percentage. This will add a key frame with the selected velocity. What you do next is chose the part of the video where you want the speed to be fastest or slowest and you decide its speed and add another keyframe using the same steps. What this will produce is a smooth acceleration or deceleration.