Use Edge Forcing

A filled cell near the edge can force additional cells to be filled.

Consider a ten-cell line with the clue 5 and a known filled Cell 3:

· · ■ · · · · · · ·

The block of five must include Cell 3. It may begin in Cell 1, Cell 2 or Cell 3.

Possible placements:

■ ■ ■ ■ ■ · · · · ·
· ■ ■ ■ ■ ■ · · · ·
· · ■ ■ ■ ■ ■ · · ·

Cells 3, 4 and 5 are filled in every possible placement:

· · ■ ■ ■ · · · · ·

This technique is sometimes called edge forcing, wall pushing or gluing.