There are two ideas that can usually help a guitarist who has trouble with bar chords. One, it is not how strong you can squeeze, it is how effectively you use gravity. Two, it is not how hard you squeese, it is where you squeeze Let me expand. Try playing the following bar chord with out using the thumb on your Left Hand. (if you are saying "huh ?" let me elaborate, if you don't use the thumb you will have to use your arm weight and strength.

G mi 7

When you are able to do this you have tapped in to the secret of barring well. Don't just use your hand strength, use your arms weight and let gravity do some of the work. Combine this force with your hand strength and you are on your way. N.B. I don't mean you should always play bar chords this way, it is simply a way to tap some unused force.