Look for recognizable patterns that indicate definitively safe squares or definitely mined squares. For example, look for a "3" along a straight row of unrevealed tiles; all three adjacent unrevealed squares must contain a mine.

If a numbered square has all its adjacent mines flagged, use your right mouse button to click on the numbered square to quickly reveal all adjacent unmined squares.

If you're stuck in one part of the grid, go work on another part of the grid; you might eventually reveal more squares in the area where you were stuck.

You can never lose the game by guessing when there's no safe move to make. But if there's a single safe move - where a non-mine square can be identified with 100% certainty - then you might click on a mine if you guess!