Squirrel hunting with a squirrel dog is the absolute best. If you know anybody that has a squirrel dog, get them to take you. If you like squirrel hunting, consider training yourself a dog - terriers make great ones.
When you see a squirrel disappear into a tree and you are looking for him, look for his eye. The eye is a black perfectly round spot and once you get the knack of spotting the eye you will zero in on the squirrel much faster. A squirrel will generally get up into the tree and lay flat along a branch, but will try to keep you in sight with as little of his body showing as possible. Look for that eye.
Second best thing to look for is the tip of the tail either twitching or moving from a breeze, that motion will often attact your attention. An older than hills trick that sometimes works is to keep some rocks in your pocket. If you know the squirrel is up in the tree you are looking at, but you can't see him after a while, flick a rock to the other side of the tree - don't make a big arm movement doing it. Sometimes it will startle the squirrel into moving around to your side, giving his position away.
And never never never shoot into a squirrel nest.