If I have a long cut:
I cut the front edge of my vinyl roll straight. (This is real easy with Oracal, since their backing paper is a grid.) Then, I align the vinyl with the teflon cutting strip on the plotter. Pretty much what Flame does, by the sounds of it.
&
I slow down the cutter from the usual setting. It prevents the vinyl from flicking up at the back quickly, which can cause errors.
That's about it really. I'll cut 5m (16ft) no probs, and 10m (32ft) if I'm feeling brave. heh.
The length you slice a design in half really depends on what size piece you can install on the window at once. (Are you installing by yourself, or with someone elses help?)
If you decide to install on your windows using the wet app. meth. - the application tape
can be a little tricky to remove (esp. small letters where there is not much hold on the glass). Dampening the tape (once the vinyl is in the correct position) and leaving it for a minute or so will soften the app. tapes glue, making it easier to remove.
Oh! And you guys forgot the most important tool! The trusty squeegee!