First off you need to locate their nests. (Just incase you don't know what they are) You see those big balls of sticks and leaves up in trees? those are their nests.
Find a place that has several of these and hunt by them. In the morning they will be coming out and evening heading in.
You know you are in a good area when you see lots of these nests.
You need to stay longer than 20 minutes. Things are just begining to settle down in that short time.
Here is a cheap squirrel call, costs 2 cents.
Take two pennies with you when you go next time. The reason is, when you walk in the woods, your presence causes animals to go into hiding. it will take awhile before thing return to normal (as mentioned above)
When you find a stand sight and get settled in, wait about 15 minutes. Take out the two pennies. Turn one over to the tails side, so you can see that monument on the back. Take the other penny a slide the edge of it long the ridges on that monument. This sounds like a squirrel chewing on a nut. This sound will help return things back to normal and will help relax squirrels and let them think all is normal again. They will start moving again.
Those misty rain/drizzly type of days are actually good days for squirrel hunting. They seem to like a fine drizzle and are usually active most of the day. This is also a good time for you to still hunt and move quieter.
If you happen to spot a squirrel at a distance and then get to where he was at and don't see him, this is a good place to try that penny call I mentioned earlier. Be sure to wait about 15 minutes before calling. They need to think you are gone.