Summarize:

If you're hunting on a small body of water where it's not too windy, choose a duck call that isn't too loud, or you'll scare the ducks. A double-reeded wooden call is perfect for the job. On large bodies of water or when it's windy, you need a loud duck call. An acrylic call would be more appropriate. If you only have one call, vary your calls to compensate. Remember: accuracy is the most important thing. Watch the ducks for reactions to your calls. Calls are to be employed when you've spotted a group of ducks flying overhead and you want to attract them to land as close as possible to your position. Calls are most effective when they're used sparingly, and they must be done correctly if you hope to fool the ducks.  Watch the ducks for their reaction to your calls. If you see them flying over-head and change their direction to your position, don't keep skronking away at full-tilt and risk ruining your cover. Wait for them and see what they do. If you're calling more than once in 30 seconds, you're probably calling too much. If you're listening to Waylon on the portable radio super-loud, your whistles will make poor accompaniment. If you're hunting with decoys and the ducks are clearly attracted to them, don't risk ruining a good opportunity by blowing your lungs out. Ducks will often swoop around, dive, leave, come back and land several times before they decide to jump in front of your sights. Be consistent, avoid frustration, and wait. Listen to commercial CDs of duck calls. Listen and practice at home or in the car. Also spend time listening to real ducks in the wild. While you're calling, you should be actively listening to the sounds the real ducks are making so you can imitate them in response. Wooden duck calls especially will need drying out and wiping down after use, so the wood finish won't wear and crack.  Unscrew the reeds and check to make sure they haven't broken or chipped, which will affect the sound of your calls. If they have, replace them. Before you remove the reeds, use a felt-tip pen to mark how far down the reeds are screwed into the call so you can replace them at the same depth. Misaligning the reeds will result in a changed tone and make it difficult to reproduce your calls.
Use the appropriate call for the occasion. Call sparingly. Eliminate other distracting sounds while you're hunting. Don't call if the ducks seem attracted to your decoys. Be patient. Practice. Clean and tune your duck call after using it.