As others have said, practice. One bit of advice I can give you is this, hold your gun over or under the target, bring the gun down or up as the case may be, and be confident enough to fire instantly you have the target in the cross hairs (if using a scope that is). With free hand shooting, the longer you hold aim the more likely you are to miss. And as Timelord says, it's a true discipline.
Stick at it mate and good luck.
Rob