Holding the <Command> key down while clicking on a square will highlight all the surrounding squares which have not been uncovered or marked.  It will also uncover all of those squares only if the center square has been uncovered and the number of marked squares around it is exactly equal to the number displayed by the detector.
The final tool you have is your own brain.  Look for numbers which have the correct number of mines marked around them.  This is easiest with the smaller numbers.  After a while, you will start to notice common patterns. If you need some help seeing this, select 'Hint' from the Help menu.  If the program finds a square which you should look at it will flash that square.  A flashing square means either that number of mines are flagged around it, or that there are only enough unflagged spots around the square to exactly satisfy it.

If you reach a point where logic does no good, you can get a hint or two.  When 'Uncover Safe Square' is selected from the Help menu, a safe square will be picked and uncovered.  Use these only when you really need them; you only get a few per game.