shim your Transmission and differential .

What you will need a assortment of 3 mm shims and 5 mm shims in various thicknesses 

All this is trial and error and takes time to complete but you will be rewarded with a transmission that will last much longer than if you had not done this  If you have a brushless model or plan to upgrade to a brushless system I consider the differential shimming manditory.

1. Remove your transmission from the car an remove one side cover.(If you still have bushing's install bearings now). 
2. Then remove the differential and idler gears from the case.
3. Place one shim on the input shaft.
4. temporarily reinstall the side cover with a few screws tightened snugly try to slide the input in and out  to test for play  if it doesn't move rotate to test for binding.
5. If rotation is stiff dissemble and try a thinner shim . If shaft slides in in out freely add a additional shim . experiment with different thicknesses till shaft rotates freely but doesn't move in and out of the case..
6. Re install idler gear with one shim re install side cover as before but with a few more screws and test for binding . If binding is present try a thinner shim . If no binding is present try a thicker shim or install a second shim .    

Now lets look at the differential gears mine had lots of play .

1. Well after you remove the 4 screws on the side you will be greeted with this .
2. remove the center bevel gears and proceed to remove the e clips from the out-drives.
3. Pull the out drives from the case halves and add a 5 mm shim to one as seen in picture number 2 
4. Add and remove shims as necessary there should be just a light bit of resistance to rotation and installation of the e-clip should not be difficult .
5. With slop removed from the out-drives the only thing left is the play between the gears themselves .
6. Start by installing one 3mm shim behind each gear as in picture number 3 , both shims must be of the same thickness.
7.  Reassemble with just a pair of screw to test for play . If binding is felt remove shims and try a thinner  shim or no shim at all . 

I should also point out I added a o-ring over my out-drive as can be seen in picture number 2 to help protect the bearing  It fits perfectly in the gap between the out drive and the case .

Here is a video that may help you understand what I'm going for a little better.