You should floss your teeth every day to reduce the incidence of tooth decay and gingivitis that leads to more serious gum disease. Here's one method that works well.

Step 1: Select Floss

Research indicates that any style floss is equally beneficial for plaque removal and gingivitis prevention when properly used.

Choices include:
waxed - best for rough surfaces/restorations in the mouth that snag unwaxed floss
unwaxed - thinner, slips between teeth and under gum tissue easily, flattens out against the tooth, and makes a squeaking noise when tooth is clean
Kevlar - strong and slick, resists tearing on rough surfaces/restorations
braided - stretchy yarn-type floss especially good for large spaces between teeth

Try them and see what works best for you. The important thing is that you do floss.

Step 2: Cut Floss

Cut floss into a manageable length, about 18 inches.

Note: we'll be using black unwaxed floss for better contrast in this Instructable. The color is otherwise irrelevant.

Step 3: Wrap Floss

Wrap a few turns of floss around the middle finger of your dominant hand, then wrap the rest around the middle finger of your opposing hand. Begin wrapping at the loose ends, and work your way towards the center of the floss.

As you use the floss, you will unspool clean floss from your off hand and wrap the used floss onto your dominant hand.

Step 4: Finger Positions

You will use two fingers to manipulate the floss inside your mouth. Press against the floss with the pads of your fingers, and direct it between your teeth as needed. Change up your grip as necessary - here are some tips to start out.

Both index fingers: good for reaching lower molars
Both thumbs: good for reaching upper molars
Thumb and index finger combination: good for upper and lower front teeth

Step 5: Insert Floss

Carefully insert the floss between your teeth with a light sawing motion. This is the only time you'll use a sawing motion - once the floss is through the contact (the tight junction between adjacent teeth - you'll likely feel a slight snap as the floss clears the contact) avoid sawing, as this can cut and damage the gums.

Slide the floss gently against the tooth surface up and under the gum line until you feel a light stop. This is a critical area to clean with the floss, but always be gentle - forcing the floss too high under the gum can cut and damage the gums.

If you haven't flossed recently or your gums are inflamed, they may be painful or bleed lightly. Don't stop - this indicates a need for more (gentle) flossing. If there is significant bleeding consult your dentist for a more thorough cleaning. Well-cleaned teeth with healthy gums don't bleed.

Step 6: Floss Teeth

Curve the floss against the tooth surface, maximizing surface contact between the floss and your teeth. Rub the floss up and down against the tooth surface, sliding from the tooth contact point up into the gum line and back.

Work the floss against one tooth then against the adjacent tooth, shifting your grip and direction as you move to the next tooth surface. Be careful not to damage the gum tissue between the teeth (called the papilla) as you shift the floss.

Pop the floss back out between the tooth contact, removing any dislodged debris, and move along to the next spot. Any remaining debris may need to be pushed out with a tool like a Stimudent, Proxibrush, or dental irrigator.

It's best to start from one spot and work systematically through your mouth to be sure you don't miss any teeth. Be sure to include the back side of your last molars as well.

Note: some adjacent teeth may not actually make contact. Be extra careful cleaning these spaces, as they are particularly prone to collecting food. You should consult your dentist about any open contacts, as he or she may advise closing these open contacts.