Always look in the corners - often some of them can be deducted from the very start.
Never forget: L-shapes with the same symbol cannot touch! This will actually help you in unexpected ways to solve the game.
You are encouraged to draw any bit of information that you can deduct. Just part of the L-shape such as an edge or corner, without knowing anything further. The more the difficulty increases, the more even such small indications become useful.
For example: on adjacent cells with different symbols you can draw an edge between them. For adjacent cells with the same symbol, you can draw two edges outside of them. When to two diagonally adjacent cells have the same symbol you can draw the corners of the L. (illustrations)
Identical symbols that are one space apart, cannot belong to the same L-shape, and since they cannot touch, the further restrict each others placement - so more edges can be drawn!