You can use this module with the following in your ~/.xmonad/xmonad.hs:

 import XMonad.Layout.MagicFocus

Then edit your layoutHook by adding the magicFocus layout modifier:

 myLayouts = magicFocus (Tall 1 (3/100) (1/2)) ||| Full ||| etc..
 main = xmonad defaultConfig { layoutHook = myLayouts,
                               handleEventHook = promoteWarp }

For more detailed instructions on editing the layoutHook see:

XMonad.Doc.Extending

An eventHook that overrides the normal focusFollowsMouse. When the mouse it moved to another window, that window is replaced as the master, and the mouse is warped to inside the new master.

It prevents infinite loops when focusFollowsMouse is true (the default), and MagicFocus is in use when changing focus with the mouse.

This eventHook does nothing when there are floating windows on the current workspace.