Technique matters, but so does mic placement. Too much sibilance occurs when the mic is placed in a position where it is getting a lot of the air from your mouth but not enough of the resonance from your chest cavity.
If the mic is straight in front of you, you're going to get bad sound no matter what mic you use, even with a fairly good singer—not just sibilance, but also pops from plosives, etc.
If the mic is above the singer, you're going to get a thinner sound that's more neutral with respect to sibilance and plosives. For a female voice (and particularly a high female voice), the first thing I'd do is to try putting the mic above the singer, pointing downwards, about two or three feet away.
If the mic is below the singer, you're going to get a deeper, thicker, fuller sound, again with less emphasis on random noises. For a male voice, the first thing I'd do is to try putting the mic about 18 inches away at chest level, and having the singer sing over the top of it. Point the mic upwards towards the singer's throat.
Probably the worst thing you can do is to put the mic a few inches away and directly in front of the singer, pointed towards the singer's mouth. That tends to emphasize all the wrong things. This is one reason why live performances so frequently suck.
In either case, do not under any circumstances allow the singer to aim his or her voice towards the mic.