Usually, it is said aliens somehow had to look like animals on Earth, or monsters of legends, so that people can "relate" to them. Like a green tiger with six legs: It's still a tiger, so people know what to expect when it suddenly pops out of the hedge.
Also, in the movie 'District 9', which I loved, the creator made a statement in an interview that I really hated. Even though the aliens in the movie were strange insectoids, he said, it was "necessary to make them walk on two legs and have two arms, so that they are people. There was no other way. If it walks on four legs, it will be a dog. It wouldn't work."
Personally, I find it very hard to believe a species evolved on a planet far away would look like your mailman. Also, I think it's kinda sad people can only feel empathy for creatures who have the same overall bodyplan as themselfs.
I didn't watch the movie 'Avatar', but from what I've seen and read, it's just the same retarded logic. Evil humans come to colonize and exploit a wonderful planet with nice blue aliens who happen to be as beautiful as angels and who live in harmony with nature. Not only do creatures have to look human so the public is inclined to take their side, they also have to be pretty. If they were giant cockroaches, no one would think destroying their world was wrong in any way. Is it a wonder that ugly people receive harder punishments than pretty people for the same crimes? And yes, that's proven by statistics.

Anyway, my point is that I have no intention to play this kind of game. If I want to make aliens, they will be aliens. People shall relate to them in the right way. So that I can say that if they cry when a dog gets beaten up, but don't when it happens to it's alien equivalent (that happens to look like spaghetti with meatballs), they are just a slightly better versions of psychopaths.
I'll do my best .

My ambition is to make something truely alien. You know how aliens generally tend to be no more then little green men (or big blue men, in case of 'Avatar' ). The most alien I've seen so far is Darwin IV of Wayne Barlowe, with the result that everyone who has read the book is trying to make aliens that look just like his. And I am not even any better. But hey, I'll try to work on it.
By Monday, my computer will be fixed and hopefully I will be able to properly scan, artificate and upload again.

When people look at alien portraits, the ideal effect would be that at first they have no idea what they're even looking at. They just see a very complex organic mass. And only after studying it will they know what to think of it. No more six legged dogs, no more plants with teeth, no goldfish with antlers, no little marsman with big black eyes.
Since one can't avoid to work with at least some common shapes, at least trump people. Drawn something that looks like a cute baby and turns out to be a mushroom. Or a mushroom that turns out to be the master of the universe.

I try to use strict rules on my new concepts. I'll try to avoid as much similarities with Earth as possible- so that you will require some pondering to discover the actual similarities.

Of course that requires a lot of creativity and inspiration, which I tend to lack. I already managed to make something like a fauna without legs and covered in something which is neither scales, nor feathers, nor anything found on large animals on Earth.
My truly alien planet won't have any plants in the traditional sense. I am not going to draw grass and trees. It was already kinda disappointing to discover that even Wayne Barlowe made the pocket forests of Darwin IV out of ' plaquebark-trees '. If I want to look at trees, I go to the park.

One of my ideas was to make "plants" floating around like fog or clouds. But that's complicated for many reasons (*what will happen if there's a storm?*). Also I wanted to explore the idea of migrating "plants". "Plants" are necessary because you kinda need something that gives a character to the landscapes.

Maybe I will really just start with random shapes, and even I won't know what they actually are, and then I decide to call them plants. Or something like that. Today when I was in the city, the garbage in the streets sure looked like alien "plant" material...