formulate the guidelines in choosing the mods
come up with a hierarchy before they reach the mod status
divide it accordingly (support, testers, officers, friends etc.)
assign key roles and responsibility
provide key performance indicators for each role
once achieved they can now be elevated.
indicators should be challenging and time consuming.
at the end of the day, you have a fair and square approach in choosing the mods
at least this is how i do it for my forums.