You can finished half works of UVs,and mirror it . this plugin work only at symmetry polygon model .
1.install mirrorUVx.0.mll to you plugins path
2.in maya,load it
3.maya6: Menu -> Edit Polygons -> Texture -> X_Mirror UVs
maya7: Menu -> Polygon UVs -> X_Mirror UVs
maya8: Menu -> Edit UVs -> X_Mirror UVs
4.a window is displayed, read info,use it!
Precision: if the model is not complete symmetry,increase it ,but don't expect good result .
filp: Filp Horiaontal '-x' to 'x' or 'x' to '-x'
Flip Vertical '-y' to 'y' or 'y' to '-y'
offsetU,offsetV:if you try it ,you can understand. :-)