Many AIs seem to weight phrases in the prompt based on their location within the prompt - that is, the earlier mentioned, the more important. As such, place the most important descriptives up near the beginning of the prompt. For example, "a green-skinned humanoid with pointed ears wearing a military uniform" is more likely to get you a Vulcan, perhaps not in uniform; "a military officer with pointed ears and green skin" is more likely to get you something that could pass as an Imperial officer, who may or may not be able to pass as Darrian or as Vulcan.
(It will also depend on which generator you're using; Wombo doesn't seem to do people as well as NightCafe, for example.)