Fantasy writer Tessa Hei uses a world-by-world basis. Her approach is simple: use magic. Many times, her work classifies as urban fantasy, which means it is set in a modern-day setting, but with the addition of magic. If this sounds a little like your world, then read closely. Tessa Hei has a simple scheme to help develop her worlds.
She calls it "Hidden/Unhidden". Hidden is when only a few people know of this magic or supernatural entity. Many paranormal romances fall under this category. On the other hand, Unhidden is when everybody is aware of the magic. As Tessa puts it,
The difference between "Hidden" and "Unhidden" isn't the amount of time the character is aware of the magic; it's the amount of people in the world who knows about it. If it's lightly spoke of among anyone (e.g. you can say it to your grandma, the bag lady down the street, or the graduating class of '09), it's generally Unhidden. If magic is known but outlawed (not spoken lightly of), then what? That would be an Unhidden world; it's common knowledge. If magic has been forgotten and rediscovered in medieval ages: Hidden; not the whole world knows of it.
My approach is different. I focus more on the race itself, picking out points which could be used to differentiate humanity from an alien or supernatural species. The main points I concentrate on are: speech patterns, customs, and prejudices.
Clara just finished talking to you about language and slang, but I'm still going to skim over it lightly. When I work speech patterns into my races, it involves more than just slang. First of all, you do have to watch that your aliens DON'T speak slang, not unless they've been immersed in human culture for a while. Then, I always keep in mind that whatever my race, if their first language is not English, they're going to approach speaking English in a unique way. They might structure it differently. If any of you speak a second language (or if English is your second language) you likely know what I mean. Living in Canada, I speak some French. My first instinct is always to structure it as I would in English, but using French words. That is not always correct. When it doesn't jar the reader, I try to work in the same speech patterns with my alien characters.
I'm going to move on to prejudices and leave customs for last. Think about what prejudices afflict humanity, either now, or in the past. The main ones are: race, religion, and gender. Sometimes I work those into my stories. More often, I like to come up with new ones. Age, for instance. Either the young are dismissed out of hand, with the old being honoured almost to the point of being revered; or the old are dismissed out of hand and the young are honoured to the point of being revered. If the latter case, you could take the prejudice one step further, to where the old are often prosecuted or killed. Or, I once wrote a story where the status in society revolved around the number of eyes you had. The fewer eyes a character had, the higher up the command ladder he or she was. The same might be said of arms, legs, spots, horns, or wings.
Now, on to customs. This is where I like to show the divide between humanity and the race I've created. Even if I was given a thousand pages, I probably wouldn't be able to name all of the possibilities. In terms of romance, major customs you might want to explore are: mating, marriage, the naming of children, courtship, and certain things like fashion, eye contact, touching, dancing, etc. which might inadvertently encourage a sexually agressive response from your main character or love interest. Other ways customs might differ would be in burials, education, showing respect, or the use of body language.
In the end, it depends on how you'd like to make your point. If you're trying to illustrate how different the two species are, then I suggest you work in as many points of difference as you can. If, on the other hand, you're trying to show that the two, although different on the outside, are really quite similar, I would start with some differences in the beginning of the story, but weed them out as you go along. If a character was once human, likely there won't be very many differences at all.
Thanks, Clara, for having me!
L. K. Below blogs at lbelow.blogspot.com. Her paranormal romance short story “His Familiar Touch” is set to be released in Cliffhanger Books' Paramourtal anthology. Visit her at www.lbelow.com.