Clamping your workpiece and guide down adequately is key to getting an excellent straight cut. Have the side of your material which you require the best finish on facing down to prevent splinter tear-out on it. Applying painter’s tape to the surface of the material and cutting through it can also help prevent tear-out.
Set the blade fractionally deeper than the material you are cutting, so it is only just protruding. Hold the circular saw firmly and apply slight pressure towards the side with the guide to make sure it tracks correctly along it. It's best to cut the full length in one smooth motion without stopping and starting the machine. I recommend a thin kerf 60 tooth blade in a 180mm circular saw for a very clean and precise cut. - MitchellMc
I don't have a table saw or mitre saw to make the cut and I don't think a circular saw could make a clean cut over a long length. I've heard of people clamping a guide on top of the sheet and using this but haven't got any feedback on its success (or failure). - Willow
There is something you can do if you have a circular saw. The saw itself is important however what's more important is the blade. There is a simple rule to follow: the more teeth the blade has the finer the cut.
For a straight cut, what you can do is get a straight edge piece of timber (that's as long as the cut), calculate where you want the cut and clamp the timber down and use it as a guide for the side of the saw. - Dan