Dog cannot be afraid of slippery surfaces. Dog must be healthy. If not, the dog may be returned to you. If he's a confident dog, this is not a problem. Severe dog-aggression will usually rule him out.
Officers prefer male dogs.
If a female is offered up, she must also be a social animal and have that ball-crazy drive at an extremely high level. Most females are not taken into the programs, but if they are, they are more likely to go into narcotics-sniffing than K9 patrol units.