How do I decrease the slop of an axle?

I know you should use at least two bearings per axle but is there anything else I can do to decrease the slop?

use a screw if possible, and keep the two bearings

So the first question is where the slop is.

between 2 meshing gears?
between the axle and a gear on the axle?
between the motor and the axle?

The axle is going through a 60 tooth gear which is meshed with a pinion gear. The axle which is going through the 60 tooth gear has lots of slop.

Shorten the width of the axle/gearbox if you can, use screw joints wherever you can, and Just make sure that nothing is loose