FOOD
We started out with one cedar feeder hanging in the backyard.  One day we were surprised to see a gray squirrel hanging by his toes from the top of the feeder and reaching down toward the opening.  He began swinging the feeder so the seed would spill out for his squirrel buddy waiting below.  They ended up chewing the sides off the feeder and all the seed ran out onto the ground.   We have since retired that feeder and use a "squirrel-proof" hopper feeder in addition to several ground trays, hanging tube feeders, suet feeders, and hummingbird feeders.  The battle with the squirrels is on-going and it's pretty hard to discourage them.  The debate goes on about whether or not to feed them.  I must admit that when there is several feet of snow on the ground and they look in the window so pitifully, I pull out the peanuts and the squirrel feeder and set it up.  (They are awfully cute and very amusing to watch.)
There are a lot of good resources on the best types of seeds (see resources page).  The types of food you offer will of course depend on the species you have in your area.   See "Feeder Chart" to see what works for us.
This is our "squirrel-proof" tube feeder.  The squirrels eventually found out how to lift off the top and fit in between the cage and tube.  There is only about two inches of space but they managed to squeeze in - they love the black oil sunflower seeds.  We now have the top and bottom fastened to the cage and use safflower seed - they don't seem to like that as much.  This is probably our most popular feeder - it is off the second floor deck and draws a lot of birds.