There is no question that medical masks and other PPE (eye protection, etc.) are effective:
A review in the Lancet(external link) showed that transmission was lower by up to 82% with physical distancing of 1 meter or more, and was lowered as distance was increased. Medical face mask use (N95 or surigcal masks) could lower risk by up to 85%.

Homemade Masks

I have been skeptical of the value of homemade masks based on past research that suggested they were far less effective than medical masks, but I am modifying that opinion based on more recent research:

A study in Vietnam found 6 times the influenza rate in healthcare workers wearing a cotton cloth mask as those in a medical mask.

Now, however, there is increasing research on materials and techniques used in construction of homemade masks

Materials(external link):

Some household fabrics look as efficient at blocking the virus as surgical masks, and potentially more breathable.

Combining materials(external link):

Tightly woven high thread-count cotton (for instance a 600-count pillow case) is more effective in blocking large droplets than looser weaves.
Fabrics with electrostatic charge (such as silk and chiffon) are best for blocking aerosols.

Masks made with layers of the 2 different types of materials were the most efficient, for instance 1 layer 600-thread per inch cotton and 2 layers of chiffon had a large particle and small particle blocking efficacy of 99.2% and 97% — nearly as good as a properly-fitted N95 mask.

Leakage around masks

Unfortunately, small holes or leaks around the edges of the fabrics could decrease the blocking efficacy by 50% or more, and I see many homemade masks that sit far from the face or have large gaps. One study(external link) looked at adding an outer layer of a section of a nylon stocking to hold the mask against the face, which improved the performance even of most surgical masks.

Do not touch your mask!

Finally, I see many people wearing masks adjusting them constantly with their hands, and we know from a recent study that in infected patients the outside of even surgical masks can become very contaminated with virus particles(external link). By all means wear one, but DO NOT TOUCH IT CONSTANTLY and be careful removing it(wash your hands and the mask!), and do not use wearing it as a justification to relax any of the other measures you are taking.

Cover your eyes too

Goggles or face shields that cover the eyes appear to improve protection from exposure significantly, as high as 74% in one particular study.

Recycling medical masks:

Heating a face mask can disinfect it. Moist heat appears to be better than dry heat.

Multicooker Method(external link):

  • Set a multicooker or electric pressure cooker to 149°F (65°C) for 30 minutes
  • Cover the bottom of the pot with ½ inch of water
  • Place a rack to keep the mask out of the water (binder clips can be used to elevate the rack)
  • Place the mask in a paper bag, staple the bag closed, and place on the rack. Up to three masks can be nestled together in one bag.
  • When the cycle is over, the masks should be removed and allowed to dry (about one hour should be sufficient).

A study by researchers at Stanford University found that heating N95 masks at 167°F (75°C) for 30 minutes (at 85% relative humidity) did not compromise the masks, even after 20 cycles — although these researchers have subsequently cautioned that contaminated masks should not be brought into homes.

Dry heat may damage masks(external link):

Heating SARS-CoV-2-contaminated N95 mask material in a dry oven at 158°F (70°C) showed virus was undetectable at 50 minutes, but two cycles of this dry heating caused the material to lose some integrity