This is a weird behaviour,however so many of us suffer from it. The only way out is to be strong-willed.Punish yourself in a small manner each time.Deny yourself a chocolate or any of your fav food,your favourite tv show,an outing with friends,etc every time you lie for unnecessary things. Tell yourself tht this is the punishment you get for being a liar. And stick to your decision. Slowly,but surely,you will stop lying in order to gain access to these activities.It works,I tell you. But as I told before,you have to have strong will-power in order to get through.