The first thing to remember with line preparation is that their are two issues to deal with to prepare a line, one is twist and the other is memory. Tropical lines generally have a stiffer core than coldwater lines and in general are more prone to develop twist or memory than coldwater lines. Line memory is usually caused by storing lines tight on the reel. When you strip your line onto the deck/ground you will notice it lies in tight loops on the floor, this is the memory from the reel. Place the line under your foot and grab both sides of the line giving it a firm stretch and repeat the process up the length of line. Your flyline should now lie more openly on the deck. You can also stretch the line firmly between your hands if you wish.

Line twist can occur when new lines are wound on the wrong way or when flies spin in the water. You can also develop twist when casting and this is best avoided by casting all your stripped line every 5 or so casts. Remember, line left on the ground can develop twist during the cast.
Twist is best removed by running your line out behind the boat including a portion of backing. Make sure there is no fly attached or it will twist. If you are not on a boat then try pinching line between your stripping fingers when retrieving, you will notice the twist pushing forward.