If you ever want proof that fact is stranger than fiction, I recommend watching the David Attenborough series about Madagascar.  The wildlife there is far more peculiar than most of the aliens dreamed up by science fiction or fantasy writers - giant lemurs that dance and skip across open spaces on two legs, miniature ones with huge eyes to see in the dark, chameleons, millipedes and more.   All of them are fantastic, but they have all evolved to take advantage of the environment.
    Watch many science fiction or fantasy films or television series and you'll see aliens with strange protuberances on their faces or three sets of jaws, but there'll rarely be a discernable reason for them, whereas in the animal world every feature has developed for a purpose, such as the chameleon's ability to swivel its eyes independently.
    On the whole, I'd say it's best to avoid making your aliens blue or smell of lemons solely for the sake of it.  Think of the reason why such a feature might have evolved.  Is it really necessary to the story?  At least this will help your aliens to be consistent and logical.  Consider as well what the consequences of such a feature might be.  You could use one of the strange abilities of wildlife as a starting point for a story.  The ability of cuttlefish to change colour gave me the idea for my story, 'Healthy Eating', which appeared in New Myths.
    You don't even need David Attenborough's help to get inspired.  Simply go outside and turn over a few stones - the aliens are all around us.