For my part, I've always tried to base my aliens in plausible evolution -- whatever traits they have should be logical adaptations to their environment and evolutionary history. Generally, I try to think about how a species could have evolved into its current form. For instance, the
Gaurim in my
Arachne novels have six limbs in which the middle pair are the longest, with the forelimbs being arms and the hind limbs having evolved into clubbed "tails" for mating fights. At the link, you can see the sketch I drew for rationalizing how they evolved into that shape.
Learning about how Earth species evolved and adapted can give a good idea of how life adapts and how traits are selected for and against, but at the same time, I try to think of ways that different solutions might have emerged on alien worlds, ones that didn't arise on Earth. At least, I try to put traits from Earth creatures together in different combinations. There's no reason to assume alien taxonomy would break down into the exact same categories of reptile, bird, mammal, etc. that evolved on Earth. It's more important to understand the basic principles that drive evolution so that you can apply them from scratch to a different biosphere. For instance, economy -- there's a reason we have only one or two of most things instead of nine. Evolution discourages waste. If a large species has more than four limbs, there should be a good reason for it, e.g. it's a high-gravity planet and they need the extra support. Most of my species have four or six limbs. In one of my first published stories, "Among the Wild Cybers of Cybele," I worked out a biosphere of vertebrates that had only two limbs, and adapted other attributes as needed -- for instance, some species had a prehensile tail and proboscis, while in one genus, the ribs had extended out into movable fans for balance and defense.
Often I try to think of alternative paths evolution could've taken to solve a given problem. For instance, the
Biauru, the species in my current avatar (Eldar Zakirov's cover painting for the
Analog issue containing my novelette "Aleyara's Descent"), were my response to the cliched assumption that parallel evolution would produce humanoid aliens. I wanted to come up with a species whose evolutionary history paralleled ours -- four-limbed vertebrates that evolved into treetop brachiators and then into upright bipeds as their environment changed -- yet was still as different from us as possible. So they've evolved from a somewhat reptilian or dinosaur-like stock (warm-blooded); they evolved into bipeds because their home canopy evolved to become almost a solid surface rather than because they moved to the ground; they hop like indrid lemurs rather than walking; they have vestigial gliding membranes and fleshy dorsal crests that sense air motion, display ovulation, etc.; their genitalia are where are navels are and designed quite differently from ours; their hands and feet have four digits and are symmetrical; etc.
As far as psychology and culture go, I try to approach it the same way, thinking about how the species's worldview and beliefs would be shaped by their anatomy and environment. The Biauru are evolved for vivid display and vocalization like tropical birds or primates, so they're a flamboyant, expressive, operatic people. The
Denzeuur from my story "Twilight's Captives" have symmetrical bodies where the front and back are interchangeable, so they tend to be adaptable and not see things in binary absolutes. The important thing is not to be arbitrary -- every aspect of biology and culture develops for a reason, in response to a need or opportunity. You can start with the attributes you want a species to have and work backward to figure out how they evolved, but to me, it's important to keep it in the realms of plausibility.
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