The best way to cut straight is to clamp a guide down to your material. A detailed guide to make one is listed here http://members.aol.com/woodmiser1/sawbd.htm
Basically it is a 10” wide x 3/4 plywood at what lengths you may want 3', 4’, 8’, ect. Attach a straight 1-1/2” strip of 3/8" plywood positioned to allow the blade to cut off excess. Now the edge of the guide is perfectly in line with the blade.