Back lash is typically proportional the quality and number of the physical gears use in the gear train, so avoid adding in gears that you might be able to avoid; so spend more on a increased number of teeth on your worm gear wheel, and avoid using the planetary gear on the motor that drives the worm screw.
Design stuff: My celestron scope has spring tensioners that force the gears to one side of the gear groove, and with a well balanced (slightly biased in one direction) helps enormously with the backlash. Also note that software is usually used to correct of the backlash; normally scopes include a routine at the end of a GOTO command that runs the motors in a consistent direction, thus ensuring (no matter which way the gear slewed to the target), then will end up with a +ve RA & DEC direction, which applies force on a consistent side of the gear grove. This all helps for GE mounts - but standard non-GE mounts - then backlash corrections are more problematic due to tracking DEC/RA forcign the use of both sides of the gear grooves.

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