I'm sure there will be other suggestions, but mine is to make either a sawboard or routerboard, which is just a length of 1/4" hardboard that has a strip of material with a known straight edge attached.
Use the circular saw (or a router with the bit you will use to straighten the edge) to trim the excess hardboard after attaching the straight edge piece. The sawboard/routerboard is then clamped to the material and will make straight cuts for you.
http://bt3gallery.fhauto.com/view_ph...le&id=Dsc00003
Above is my routerboard being used to line up a slot to cut to create a 'toilet-
bolt' clamp track, but using it clamped to the edge of your existing cut would allow you to straighten it out.
Below is a sample of a sawboard.
http://bt3gallery.fhauto.com/view_al...e=Sawboardpics
Note: Be sure to always use the same bit in your router once you have trimmed the hardboard for it's use.