Tips and tricks

 Mask modifier

 Mask modifier is very useful when modeling / rigging characters. You can mark a vertex group that is hidden when rendering/ in 3d view. That group completely disappears from view. It's handy tool to hide any intersecting geometry, like character's body clipping through clothing etc. It has many other uses as well. I must leave it to you to discover other formulae.

Blender checkboxes

 Blender v2.67

Did you know that you can now enable/disable many checkboxes easily. Instead of clicking through them one by one you just press LMB down over one checkbox and drag your cursor over the boxes. This works on all checkboxes I guess, from layers to material settings. You have to keep shift down to select multiple layers though but on normal checkboxes you just click mouse button down and drag over checkboxes to toggle them. This is similar to the way you can toggle layers in Photoshop for example.

Blender Shape key mirroring fix

I just figured out a way to fix one problem with shape key mirroring in Blender. Sometimes when you mirror a shape key, it is mirrored but also moves some other vertices randomly. This is likely happening because you messed up something and the mesh isn't completely symmetrical. (Some vertices that should be in same place when viewed from side aren't in same place)

I had such case with one of my characters. It got fixed when I selected the whole model in side ortho view (3) with x-mirror on. (soft selection has to be off for it to work) Then I pressed S to scale and all vertices which weren't aligning properly snapped perfectly to right places on both sides of the model. Confirm with LMB. Then the mirroring operation worked perfectly.  At least in this case.

Obviously there are many other reasons why mirroring can fail, but maybe this helps someone who has such problem. Not sure if it fixes any asymmetrical vertices on X-axis.

Checklist:

1. Model your model so that it's perfectly symmetrical and don't mess it up. (!)
2. Check that the model is aligned correctly; when you go to front view you should see the front of the model etc...
3. Model's pivot should be in the center of the scene, and the model should be centered on it's pivot.
4. Apply scale, rotation, position (ctrl+a)
5. If mirroring fails try to fix with the method I mentioned. 
6. If it doesn't work, good luck with getting it to work.
7. Model all shape keys individually. Actually this is good idea if aiming for realism since nobody has perfectly symmetrical face. (Obviously you can edit the mirrored shape keys if you got them to work to save time.)