Before you do anything to the mouse, unplug it! If you leave the mouse on, you can risk clicking on random things or worse, frying out the circuitry with cleaning liquid (if you choose to use it).
Step 2: Clean the clicker areas
First you want to clean the top of the mouse where your fingers rest. Mine is usaully really gross because I eat at my computer. To clean it, simply grab a rag, or an old sock, and give it a good wipe. If the surface of your mouse is glossy, it should com off pretty easily, but if it doesn't, you can use a little water. If it is really stuck on, put a few drops of water on it and let it sit for a few minutes, then come back and try to scrape it off. Finger nails for for stuck on messes as well.
Step 3: Clean the bottom
Next is the pads that help the mouse glide smoothly along the desk. As you can see, mine are pretty gross, we will fix that. These are usually pretty easy to clean, just scrape them with your fingernail. Once you get the chunky stuff off, just rub the pads with that rag again and it should glide a lot smoother. You should do this to all of the gliders, mine has 5.
Step 4: Cleaning the optical sensor
Now, look into the hole on the bottom of the mouse, this is where the optical sensor is. If you can't see any hair or dust in there, leave it alone, you don't want to damage it unnecessarily. If it is dusty though, you can use a Q-tip to give it a quick wipe.