Boyles law: States that at a constant temperature pressure is inversely proportional to Volume.
Errors: percentage error on the pressure gauge and the measurment of the volume.
Accuracy: To improve accuracy do the experiment quickly so as to avoid too much handling of the apparatus and to minimise collisions (which cause heat) between gas particles under pressure.
Release the pressure slowly which should reduce the buildup of heat.
Allow some time between each measurement, to let the apparatus return to the start temperature.
Anything that causes a temperature change will effect the results of the experiment.