Barrier coverage is a critical issue in wireless sensor networks for various battlefield and homeland security applications. The goal is to effectively detect intruders that attempt to penetrate the region of interest. A sensor barrier is formed by a connected sensor cluster across the entire deployed region, acting as a "trip wire" to detect any crossing intruders. In this paper we study how to efficiently improve barrier coverage using mobile sensors with limited mobility. After the initial deployment, mobile sensors can move to desired locations and connect with other sensors in order to create new barriers. However, simply moving sensors to form a large local cluster does not necessarily yield a global barrier. This global nature of barrier coverage makes it a challenging task to devise effective sensor mobility schemes. Moreover, a good sensor mobility scheme should efficiently improve barrier coverage under the constraints of available mobile sensors and their moving r...