Freezing Transformations, basically sets your geometry to 0 0 0 in Translate, Rotate and sets the scale back to 1 1 1. When you highlight the model and press W on the keyboard, you will see the Move Manipulator/Tool. This should ideally sit at the origin (the thicker black lines on the grid). If it is then great, if not press insert and hold down X and move the tool until it lock onto the origin (This is best down in top or front view). Pres Insert again and you have now moved the pivot point.

Now duplicate the model and set the Scale X to -1, you now have the 2 sides symmetrical. Select both and combine them, once that's done it becomes 1 piece of geometry. Go into component mode and select vertices's. Now select 2 verts and merge. continue all the way around until all the geo has been merged. Download the Cartoon Dog tute from here (Free). Kurt goes into this type of operation quite extensively.