Should I answer every message I get on online dating sites, even if I'm not interested? If so, what's the best way to do respond?

Yes you should, though it's ok to use a cut-and-paste approach. Be as nice and courteous as you'd like them to be to you. If you've said you're looking for someone within ten miles of London, but someone answers from Australia, it's perfectly ok to say "Thanks, but I'm really looking for someone who lives closer to me." And people won't be offended when told that. You can also say "Thanks for answering, but I've already found someone." Don't poison the well for yourself and others, you might end up back on the dating site in a few months, and people you snubbed before might start looking good, but if you treated them badly, tough luck.

You should take the time to respond with a Thanks but no thanks type comment. You don't need to give details at all. Remember that not every man sends messages to lots of women. Think of how you would feel if you sent a message to someone you really liked and they ignored you.

If you don't answer, that IS an answer.

I agree with Shelly. Just be polite, and say what's really on your mind without being too detailed. For instance, "Thanks for the message, but right now I kind of think we don't really have too much in common". That sounds a lot better than saying you're not attracted to him, and doesn't make you appear to be gloating about all the messages you are receiving.

I am a lot like you. Besides, people you actually would be interested in might feel more comfortable approaching you IF you have a good response rate. I know sites like OkCupid track that. You can respond to someone without leading him on.