Load your media
Put a piece of masking tape on the heater platen, under the media so that the tape sticks out, put a mark on the tape at the edge of your media.
Use the control panel to feed out about 5 feet of media.
Your media should not skew from your mark.
If it does:
Load your media and drag out a couple of extra feet.
Put a glove on and apply a little tension on the media on the front heater.
Reach over the machine and roll the media back on the roll from the media holder hub or bar, this should straighten the media out.
If it only happens on long runs AND only when you use the take up:
Cut the corner off each side of your media, put your tape on the point and tape to the take up reel. This is your best change of getting it centered and to pull evenly.
There is an adjustment on the take up if the take up is pulling the media one side or the other.