I get asked this question alot & i respond with my own question. what foundation are you using?
There are a few types of foundations; liquid, cream, gel, mineral, powder, ect. & the type of foundation you're using counts when deciding what type of application you should use.
GEL: I used to tell everyone to throw away any sponge they own but i purchased MAC studio sculpt foundation a few months ago & i realized that it was applied best using a damp sponge. Now the reason i was so against sponges is because they collect bacteria like nobody's business. Each sponge should be used once, twice at most. Use one side one day & the other side the next, then THROW IT OUT. It soaks up alot of product as well, which contributes to excessive bacteria growth but can waste your money. If a sponge is used hygenically then i definitely recommend it for a gel based foundation. Gels do not work well with a brush. Dampen your sponge with either water or fix+ by MAC & dab the product onto the sponge & apply to the face. Use a rolling motion across your skin for the most natural finish. Gel foundations should be set with a powder.
LIQUID: I prefer using a 190 brush with liquid foundations or a brush similar to it. Some people have problems with this brush streaking but i seem to have no problem with it on either myself or clients. My fix for streaking would be to take a sponge & the dab over those areas. Use this brush in downward strokes, you want to follow the way the hair on your face grows. Another good brush would be a duo fibre brush, my current fave is the MAC 188 or MAC 130 (the smaller the better, the 187 i find to be rather large for foundation). This brush doesnt have a streaky reputation & it gives a flawless finish. Apply in circular motions. Liquid foundations should be set with a power.
CREAM: Cream foundations are very thick & heavy, generally a more full coverage foundation. A 190 brush would definitely make this product streak & a duo fibre brush isnt dense enough to coat the skin evenly with such a thick product. The brush i tend to reach for when using a cream foundation is a 109 style brush. Its small & very thick. It wont absorb the product or act too flimsy when applying. Use in a round, downward motion. A sponge is also quite ideal for a thinker product such as a cream or to blot areas looking thick. Cream foundations should be set with a powder.
MINERAL/POWDER: A mineral foundation is either in a loose powder form or a pressed powder form. Either form you dont wanna go near a regular foundation brush or sponge. Use a kabuki or a flat top buffer brush. These brushes buff the product into the skin creating an invisible appearance, eliminating the cake face look. Be warned that buffing product onto the skin can create micro exfoliation. Micro exfoliation is where the circular motion of this brush can lift up dry skin making it more apparent & dryer looking. To avoid this flaky appearance, stray away from buffing any product into the skin or spray MAC fix+ onto a duo fibre brush & just dab onto the dry looking areas. Mineral/powder foundations do not need to be set with a powder.
SETTING POWDER: Use a large fluffy brush like a MAC 150 & dust over your face. Avoid puffs which soak up the foundation off your face & they can apply too much powder unevenly. They also get dirty really quick & will contaminate your product from double dipping.
I DO NOT RECOMMEND USING FINGERS, ESPECIALLY ON CLIENTS. ITS VERY UNPROFESSIONAL & CAN CAUSE BREAKOUTS DUE TO OILS & BACTERIA ON HANDS. IF YOU PREFER YOUR FINGERS, MAKE SURE THEY ARE ALWAYS CLEAN. I CANNOT STRESS THIS ENOUGH. HYGIENE IS THE MOST IMPORTANT PART OF ANY MAKEUP APPLICATION! TO BOTH YOURSELF OR ANYONE YOU APPLY IT TOO!
***PLEASE NOTE that these are my own opinions, you do not have to agree with me. My professional opinion should have somewhat of an influence but as with most things, it all depends on what you yourself prefer.