For about four months I've been using a combination of raw, unsalted peanuts in the shell and black-oil sunflower seeds.  The method I describe below is working well so far.  I've caught more squirrels with this method (about 25) than I did in the previous 6 years using peanut butter.  It's probably only practical for homeowners -- I imagine exterminators have to use methods that require the fewest trips to your house. 

I use a shallow plastic container/dish to hold the loose peanuts and sunflower seeds (raw unsalted peanuts and black oil sunflower seeds).   The metal thing is a strong magnet.  The magnet is used to hold the bait dish to the cage or treadle (the tipping plate the squirrel moves to trip the trigger and close the doors). 

TIP:  Computer hard drives contain strong magnets.
I took this one from a drive that had failed. 

To get the squirrels accustomed to going to the trap I lock the trigger so it won't trip.  I use the thick nut shown above as a trigger lock.  Before I thought of using the nut I used a couple of wraps of masking tape.  I've read some people use wire.  I've also used chopsticks inserted through the wire grid just below the raised door.  I suppose you could use wire, twine or small bungees to hold the doors themselves open.  I've not used these last suggestions so you'll have to experiment with them. 
 

I begin by putting the bait dish at the mouth of the trap so the squirrels can see it better and reach it without having to venture into the trap.  The first time I put the trap in a new location I leave it this way for a week, or until the squirrels have eaten the bait for 4-5 days whichever's longer.  In my experience they will take the peanuts away to eat, but will stay at the dish eating the sunflower seeds.

Then I move the bait dish about half-way from the mouth of the trap to the trip-treadle to tempt them into venturing inside the trap.  Again I leave it until the bait has been eaten 2-3 times. 
 

Next, I move the bait dish onto the treadle.  Again, the magnet holds the bait dish to the treadle.  I leave it here with the trigger locked until they have eaten the bait for a couple of days. 

Fire for Effect

Now it's time to remove the trigger lock and start catching squirrels.  I usually get a squirrel within two to three hours after I remove the trigger lock.  I try to clear the trap as soon as possible after I catch a squirrel.  I worry others will sense the danger from the frantic behavior of the one that's caught and become afraid of the trap.  I'm not sure if this is a realistic concern, but it seems like they tend to quit coming to a given location after they find a trapped squirrel.  This may be my imagination.

After I catch a squirrel I lock the trigger and put the bait dish at the mouth of the trap again.  I leave it there until the bait is taken once, then move it half-way to the treadle, until it's taken once, then onto the treadle and the trigger lock removed.  I repeat this for a week or until I stop catching squirrels.  Then start the entire process again. 

You can see how the monitoring and frequent baiting required for this method does not lend itself to use by an exterminator. 

I've used this entire process since I changed baiting methods, so I'm not sure how many of these temptation steps are required, but because this is working I'm reluctant to change horses.