如何将特定控件的ZIndex设置为最顶层

how do i set any control to the topmost of the screen. For e.g. i have a textblock in a datatemplate or a hierarchichal data template..etc... now i would like to set this textblock to the topmost on mouseover. Setting the Grid.ZIndex value to 1 in the trigger for IsMouseOver doesn't work many times. In order to do that i set the ZIndex value for all the contols to -1 in the window. it worked in one scenario but doesn't work other times. how do i set any control to the topmost of the