In Infdev, ore frequency has been turned down a lot, you only find a few clusters per normal map size. I just dig a straight tunnel for a few hundred tiles, and mine out any veins I hit. I've got about 200 iron, 50 gold and way to much coal, so I guess it's working. Diamond remains elusive, though.
I just look around for caves. Once, I was digging out under my house (making a fort) and I found about the most enormous cave anyone's ever found. It stretched to the bottom of the map and hundreds of tiles horizontally, in every direction. I found about 250 iron, 50 gold, and too much coal to be counted. I even found some diamond near the bottom. So you can just use torches to illuminate caves and search for ore in those.
^ if you are playing indev...assuming op is playing infdev because indev has really been discussed so much that any user who has been here more then 2 months should not have ore concerns. Now infdev on the other hand, we know little about ore placement. I find that coal is everywhere in large clusters, and iron is common but not usually obvious. Gold and diamond I have yet to see. Though I rarely go down to the lower parts of the map since caves don't spawn yet.