Keeping characters different from each other can be hard. I've noticed this especially when I read a large number of books from the same author; at a certain point, some of the characters will start to blend together across contexts. As a reader I never appreciate this. As a writer I'm always on my guard.
My attempted solution - not that I can swear it won't happen eventually, but I'll do my best - is to make my own characters as grounded culturally and linguistically as I can. To think about them in terms of their genetic background, physiology, upbringing, and personal experience.
I've seen a couple of "character sheets" floating around the forums this week, where people have been asking if they have to know all these different things about all their characters, or if they need to write journals from the character's point of view. I think these things can help, but they can also be hard to do when you're sitting down to start a book. I'd say start with a general sense of the person, their motivations and goals and why these things are important to them. Then, as you go forward, just keep awareness of the different kinds of questions you might like to answer on the more subtle levels. The more you write about a character, the better you get to know them and the more nuance you can add. In my experience, for getting to know a character and how they operate, there's no substitute for writing a story from their point of view - even just starting and attempting one that will never get published. It makes you dig in more than you need to if you're just using a character sheet and looking at them from the outside.
The other thing is, don't make every character from a particular alien or racial group exactly the same. This is what I've earlier referred to as "running true to type." It's fun to have a group of people from different races, whether that be elves, dwarves and humans, Braxana and Azeans (thanks to C.S. Friedman) or the people of Sendaria, Arendia, Nyissa etc. (thanks to David Eddings). But if the belief systems of these people are entirely uncontested, uniform across the race or alien group, the story won't have all the dimension it could.
There are two ways to approach this. One is from the character direction, making sure that your characters are three-dimensional and have motives and inner conflicts and all those important things. That's certainly true of the characters from the authors I've mentioned. The other is to think directly about the character's relationship to the social group they belong to. I couldn't say whether other authors have thought about this; they may well have.
Take a social group that has a particular vocation, belief, or ideology that they are meant to follow. You end up with a situation where children of that group are being told "this is what you are like"; "this is how you are supposed to act." How do the kids then react to that? Do they embrace it? Are they resentful of it? Resigned? Subversive? Do they reject it directly? And if they reject it, do they keep some of the beliefs subconsciously without realizing it? All these are available options.
Ask yourself another question, too: what does it mean to be fortunate among these people? What about unfortunate? Even a group of poor or undercaste will have a difference between the fortunate and unfortunate among them, and so will a group of nobles. And groups like these will always have inner conflicts over things of value, which coexist with conflicts between groups.
Once you've thought through a few things like this, making characters different can be a bit easier. And fun, too!