Cutting straighter lines with a jigsaw
A jigsaw, also known as a sabre saw, is great for cutting curves and circles. But its narrow blade and slender shoe make it virtually useless for cutting straight lines unless you use a guide fence.
Make the fence from a straight 1x4. Set the fence on the board you're planning to cut, parallel with the cut line. The distance between the fence and the line must equal the distance from the saw blade to the edge of the saw's shoe (base plate).
To make the cut, simply guide the saw along the edge of the clamped-down fence. You can increase your accuracy by using a wide blade. A narrow, scrolling blade will allow the saw to drift away from the fence.
This technique works great for cuts up to about 4 feet long. For longer rips, clamp two fences in place and guide the saw between them.