The first thing you need to do in order to clean your VCR heads is to open it up so you can have access to all the dusty, moving parts. Unplug the VCR and remove the screws that hold the lid of the VCR in place. Once you have access to everything, assess the situation so you know what you're dealing with. You'll need some isopropyl alcohol, a can of air spray, cotton swabs, paper towels and a blank sheet of paper cut into one inch strips. Before you do any cleaning, locate the parts you'll be cleaning, which are the parts that the tape comes into contact with -- the rollers and the heads.
Next, blow out all of the loose clumps of dust with the can of air spray. This will allow you to pick them out by hand. Once the big stuff is gone, you need to clean three of the heads and all of the rollers. The control, audio and erase heads are all pretty simple to clean and the only heads you can use a cotton swab on. Just wet the swab with alcohol and clean the sides of the heads and rollers.
The read and write heads are what actually plays the tape, and they require a special technique. Your VCR may have as many as four heads, and they're all located on the large, round metallic drum. Professionals would use a chamois stick to do the following, but you can do the same thing with cut strips of paper. Soak one side of a paper strip in alcohol and rotate the drum in a counter clockwise rotation. Press the soaked paper onto the drum with your finger for about three rotations, or until you don't see any more dirt coming off. The reason you don't use cotton swabs here is because the fibers can come off and damage the heads. Make sure you don't move the paper up and down, and just apply a consistent and light pressure. Once you've cleaned the drum heads, you're all finished. Then reassemble and press play.