This design technique will help you to create unique creatures. Its an approach that at the same time helps keep your designs diverse enough so they don't all look too similar or human, while helping them human just enough for others to see them and emotionally connect to them. It is based off of the principles behind the "uncanney valley effect" which are as follows:

1) When one sees an unfamiliar object, he will respond though it were a similar object he is more familiar with.

2) Depending on how they compare, either similarities or differences will be more noticable: If the objects are too similar, what will stick out are the differences the unfamiliar object has from the familiar object. Otherwise, he will associate more with the things the unfamiliar object has in common.

3) If the unfamiliar object is not similar enough, the effect of the comparison is subliminal only. When the effect is subliminal, he will respond emotionally but he will not consciously be aware of the the objects he is using to compare with or why his emotional response is happening.

To design an alien, or some other creature, I try to leverage these principles to get the reaction from others (and from myself) that I want.

For example, (see A) I collected some designs of objects that were attractive, sleek, cool, powerful, futuristic, perfect, dangerous, luxiourios, etc (the feel I wanted my alien to have). Then I added in some animals that had a feel that I wanted to have my alien to have, like a snake, a dragon, and an insect (see B). Finally I wanted the aliens' face to have a narrow face, at least two eyes and a wide mouth, so I made note of this (see C). Then, I combined all these designs together (see D).

The result does have the combined "feel" of all the objects, doesn't it? I think so, at least.

Next, you might not have known what my sources of inspiration for the alien were unless I pointed them out. Some of them are obvious like the snake and the insect, but the influence of the car, spaceship and motorbike handle bars is subtle enough that you wouldn't see the alien and say "he looks like a mutated car spaceship hybrid with motorbike handles mounted on top!"

Once you're done with whatever you've made, you can try to create some practical, evolutionary explanation for them. IE, the "wings" on the side of the alien's head (see D again) can be directional sound amplifiers. Perhaps he has muscles that can angle them at the base on his scalp.

Last, you can use a similar approach to "remove" features that might remind some people of objects you wouldn't want them to. IE, the alien in image E has some of the features of the old lady in image F. With this understanding you can isolate design problems and prevent the viewers from seeing the wrong things from your character, and in this case, make this alien look less like a bauld old woman.

Image G is just another image I made to describe the concept to someone.