Boyle's Law is PV=k.
P is pressure, V is volume, k is a constant value. If you raise V, P must come down in order for K to say the same.
Another way to look at it is P1*V1=P2*V2, where P1 and V1 are the initial pressure and volume, and P2 and V2 are the new pressures and volumes after you do something to them.
This is important to remember: All of the avobe assumes constant temperature.