How do deal with advances from the unpreferred gender

Well, act neutral, you just aren't interested in that person. If you feel the need to hide why, just don't say.
Sorry, that's all I got

 

Take it as a compliment. Let them down gently. Imagine it was you being rejected, how would you want to be treated about it.

 

I don't have an unpreferred gender but I'd just react the same way, I react to everyone who I'm not interested in:

If they're friendly, reject them friendly but resolute.
If they're crossing a line (e.g. touching without you being okay with it, not accepting a no), be harsh if necessary and show them you are pissed. I hate when people don't accept a no.
Sometimes, ignoring helps too, that depends on the advance they make.

You don't need to come out to reject someone. You can just say you aren't interested, have a partner, aren't in the mood, etc.

 

I like the way Embi puts it. If you're not out, it shouldn't be different from rejecting anyone you're not interested in.

But of course, rejecting someone isn't always easy, especially if you're nervous about your sexuality. It can feel awkward; sometimes you even feel guilty and embarrassed. "Just" saying no can be difficult, especially if you're afraid they'll ask you why, but it's the only thing you can do and it does have the advantage of being efficient.
You'll want to remember that you never need a reason to say "no" to someone who asks you out; not wanting to go out with them is the best and only reason you'll ever need.

If they're a nice person and you feel bad for rejecting them -- don't. You wouldn't be doing them a favor by going out with them while not actually wanting to. You'd be doing the best thing for the both of you by rejecting them.
If they're pushy and annoying about it, and don't respect your answer, stand your ground, because they are definitely not someone you'd want to date even in different circumstances.

 

I flirt with everbody so I get this a lot. I just tell them I'm married or gay which ever seems more appropriate at the time. I always try to keep it light hearted and don't make anything seem too serious so they don't have to feel bad. I joke around a lot in all my casual banter.