I would like to duplicate one side of my model onto another side. like an arm for example. How do I mirror a arm in Maya ??

One basic trick.

0. (optional: tweek, move and freeze model)
1. dublicate onesided model (or use dublicate faces for partial mirroring)
2. scale dublicate on one axis by -1.0
2.5 (optional: tweek and move model)
3. combine models to one mesh
4. merge vertices on border.
4.5 (optional: check back face culling & normals of merged vertices)
5. freeze model & clear history of model!

Tip: you can make other half by using instance with scale of -1.0
(good for mirror modeling). But for finishing you
nead non-instanced model.

/Tyrian

[Edited by - TyrianFin on May 3, 2010 1:05:00 AM]

Delete teh faces one side of the model and snap the center edge loop to 0 on whatever axis you're mirroring, in the polygon menu click Mesh> Mirror Geometry, (you may need to adjust the options to mirror across the corrct axis)
If you have problems with merging vertices or spacing then open the channel box and adjust the polyMirror inputs. (often the threshold is set to high)

good luck,