I had posted this some years ago and decided to re-post it. Many artists are now recording in their home studios and I've been having conversations lately regarding microphones and pop filter placement when recording vocals.
Here is the post with updated links and pics:
Always use a pop filter when recording vocals. It should be placed at least 4 to 6 inches away from the microphone. Never place it directly on the windscreen of the mic as this will defeat the the purpose of the filter. A pop filter is necessary to prevent sibilance (Ssss sounds), pops (b and p sounds), blasts of air and moisture from hitting the diaphragm of the mic. This will degrade the sound of the vocal and possibly damage the mic. It will also alleviate the proximity effect (When a mic is placed too close to a sound source the low end response is exaggerated) and the vocal will sound much more natural.
Another reason to not place the pop filter too close to the mic is that the artist tends to move around a little when performing and the vocal will change greatly in level and tone if they don't stand perfectly still and keep the same exact distance on every syllable or phrase (No artist can deliver a great performance this way). Movement affects the sound less when the mic is a foot away from the talent. There's also less chance of microphone overload from too much level. Besides, we don't listen to singers or instruments by placing our ear directly next to them.
A quick way to find the approximate distance to place the pop filter without using measuring tape or ruler is to use the length of your hand as a guide. Place your index finger near the diaphragm and extend your thumb so that your hand makes an L shape similar to the image below:
Then place the pop filter at that distance.
Here are some links to various pop filters: