I don't have much problem with feedback. You need to set the mic so that you must be close to it, as Dave said. If low end is feeding back, you need to roll off some of the lows on the mic. Most people have too much low e.q. on their mics anyway, and it gets muffled.
Point the end of the mic directly toward you mouth. It irks me to see people try to sing over the top of it, like they are licking the side of an ice cream cone. This defeats the characteristics of the mic.
Each mic may have to be adjusted quite differently. I have a cheap cordless mic (Nady), that I must turn the high e.q. way up and cut the bass way down to make it sound acceptable.
DonM