Though I'm not much of an anchorer, I have been taught by others the proper technique.

First, start off with two Navy style anchors(forked flukes and stem), and attach at least five feet of chain to each anchor. Second, lay out your bow anchor first well beyond the spot and pull it tight to the bottom. Then troll back and lay out the stern anchor. The ratio is 3 foot of rope for every 1 foot in depth to hold adequately. If the surrounding water is 20ft deep, try to set you anchors about 100 feet apart.(Pythagarium thereom may apply..LOL) Mark your rope with different colored tape in 3 foot increments to set up accordingly. Troll the boat back towards the bow anchor line until you have the correct amount of line out to assure a proper setting.

Not only will this set-up hold you on a spot, it will allow you to play a hooked fish under either anchor line if they run, and place your anchors far from the area intended to fish.