How to get him
The ultimate aloof, ungettable guy. The thing about Ranma is that he's a total bastard. He's perfectly willing to take advantage of your feelings for him, he'll even pretend to like you if there's something in it for him. Make no mistake, it means NOTHING. Your best way is to beat him at his own sneaky game. Shampoo has the right idea, she just hasn't found anything that works.
And of course, if you're not even ENGAGED to him you really have no shot.

Competition
Shampoo, Kodachi, Ukyo are perfectly willing to kill for him, so watch your back. But your biggest threat is definetly Akane, because he actually loves her, you see. Frankly,I'm not exactly sure if there's anything you CAN do about her.

He likes you if...
It'll be very hard for you to tell, since he'll go to any extrememto hide his feelings. He's a sneaky weasel. Try to catch him off guard.

Little note about this guy
Don't even think about it unless you are a VERY open-minded person. Ranma is actually a perfect match for you if you're bi. He himself is straight, but I'm sure he wouldn't mind kinky little water games. If you know what I mean. ^_-