CDC Recommendations for Workers Involved with Avian Influenza (CDC.gov): Disposable particulate respirators (e.g., N-95, N-99, or N-100 (NIOSH 42 CFR 84)) are the minimum level of respiratory protection that should be worn.
All masks should be disposed of without touching the contaminated outside surface. Necessary protection also includes gloves, protective eyewear, antiviral sprays and solutions, and lots of soap and water for hand washing.  Protective clothing should be warn as well if direct contact with the virus is anticipated.
If one is buying masks it is important to realize that once used,  they are infectious and need to be disposed of properly and without infecting one's self  or others.  They should be disposed of after one wearing, or  if  wet or soggy.  Flu is an airborne infectious disease and one can be infected by viruses in the air or on surfaces.  Eyes, nose, mouth, and all mucous membranes need to be protected, but thought must also be given to contaminated surfaces. Influenza viruses can remain viable for 6-14 days and like the cold and survive in water.  This is a real infection control challenge.

For workers who are dealing directly with infectious disease such as influenza,  the CDC recommends an N-95 (or higher)  face mask with a good tight fit around the edge where it contacts the face.  These can be difficult to tolerate for long periods because they make breathing harder. They are more expensive than procedure masks.

A.  N-95 Mask: The Ultimate,  but does not make it easy to breathe. Fluid resistant and at least 95% efficient against solid and liquid particles larger than .03 microns, excluding those containing oil. (The "N" stands for Not oil proof)  The N-95 or higher is a best choice if one expects to be exposed to influenza.  The fit is important so that there are no leaks between the mask and face.  Consider smaller sizes for children. 
A " Duck-Billed" shape N-95 is available that allows extra breathing room for the face and nose.

B.  Fluid Resistant Surgical Procedure Mask:  Not optimal but an obvious second choice. These are not recommended for those working with patients infected with flu. They do offer some  protection and are more comfortable and less expensive.  High performance surgical masks  exceed 99% filtration efficiency at 0.1 microns. Again, fit is important with airborne disease.
Procedure masks that would be fine for blood splatter and blood borne pathogen protection (hepatitis B and HIV)  do not filter smaller airborne viruses like influenza. 

C. Not to be relied upon are simple dust masks and procedure masks not meeting these fluid resistant and filtration standards.  Most are not fluid resistant nor do they filter flu virus.

Do not forget that Flu virus is spread by droplets and it can live outside the body for a while.  It can be caught from surfaces (use disinfectant)  or from contaminated air.  All mucous membranes must protected (It is not clear right now whether eyes can be a route of flu spread).  This means goggles can be considered for eyes, with wrap around glasses being better than no eye protection.  Latex exam gloves are excellent protection: they should be removed with out touching the outside.  They act as a psychological barrier to  keep you from putting your hands in your mouth, eyes  and nose.  Coveralls may also be needed when one knows one is exposed. 
Disposal of used equipment without touching the infected surface is a must.  No hanging your mask from one ear, wearing it on mouth but not nose, etc. 
Masks should not be re-worn and should be disposed of without touching the outside. Hand washing follows equipment removal.

Hand washing in all circumstances at frequent intervals and at after touching surfaces is a good idea. Covering mouth and nose when coughing or sneezing, followed by disposal of the tissue is a must.  Public spitting should not be practiced or tolerated.