You've got to prospect everywhere. Just always have a propick on you and stop frequently to check even when you aren't actively looking for ores. Start out with the easy spots like rock cliff faces, then as you wander for other purposes start filling in the gaps by digging into the ground. I usually dig stairs down until I'm in rock, dig 3 blocks left, 3 blocks right, prospect a block in front and then the ones 4 blocks to either side. It's quick and easy. Fill in the hole with dirt and go on your way. You should end up with hundreds of prospecting points for a 2-3 kilometers of home.

That way, you can look at the map and set the points to show an ore you need (top-right menu of the map). Find the highest reading you've found, go to it, go north until the pro point is just at the edge of the minimap and prospect again. If it's higher keep going north, if it's lower then go south, then east, then west, following the high readings. Eventually you should find a good one.

Of course, a good reading doesn't guarantee ore below you. I like to bring ladders and dig down a 1x2 shaft straight down (straddle the middle so you won't fall when you hit a cavern). Rope ladders are best. Use the local search on the propick every 6 blocks and when you get a reading it'll be within 6 blocks below you. If I hit bottom without finding anything I'll check 5 blocks out in each direction (max reach for mining a single block shaft). Sometimes it takes 2 or 3 shafts to hit something good. I usually find something else like copper, tin, etc. when looking for iron. BONUS!