How do you cut plastic square

[gcn]

With my limited experience of scratch building one of the biggest issues I've found is how to cut plastic sheets square.

What have people found to be the best tools to use.

[Modelnut]

Years ago I bought some cutting aids from Northwest Short Line. One is a device sort of like a jig called the Dupli-Cutter. It has metal strip that holds another strip that is adjustable. You follow along the strip and you get a straight cut. 
I think Micro Mark sells tools also. 

Also invest in a GOOD metal straight edge ruler and use it exclusively for all your measurements. Not all rulers are identical. Swapping rulers during a project and things will be off a bit.

[GAJouette]

  I have a Chopper II ,but I'm not certain who makes it. Simple to use with adjustable backstops to hold the piece at the wanted measurement and the cutting is done via blade with a handle. Comes with serveral backstops for straight cuts or angled. The entire thing is mounted on a board roughtly 6" x 20". Another benefit besides square cuts is that once you're set up you can cut exactly the same lenght all day long. A very handy item for the scratch builder. As said above get a good metal rule in milimeters and a good metal triangle and square and you're pretty well set.
Highest Regards,
Gregory Jouette

[pepperman42]

Yeah - all of the above and a t-square. The good metal ruler is a must though.

Steve

[Jose Chaica]

... The good metal ruler is a must though.

... and a brand new blade! Also, a steady hand helps a lot (holding your breath while cutting, helps!).

[Pete Nottingham]

Yes I agree with a good metal ruler as a must.

Cheers

Pete.