Several katakana look similar and may cause confusion. These are:
シ and ン
シ and ン can be thought of as smiley faces looking towards the right. The character with both eyes is a shi (she), while the character with one eye has a nice eyepatch. The short strokes on the top slant towards the right, while the same strokes for ツ and ソ slant downwards.
ツ and ソ
ツ and ソ can be thought of as rating systems for waves at a beach. The short lines represent the rating (or people on the beach) and the long line is the coast. A rating of 2 short stokes means tsunami, and two people are running away. A rating of 1 short stroke means soft or so-so waves, with a single person relaxing on the beach.
ウ and ワ
ウ, the umbrella, has a ferrule on its top while ワ, the flipped over wagon, does not.
ヲ and ヨ
ヲ, the (World Wor Won), is rounded like a hand-written 3. ヨ, the gangster YO! is angular and has a small line extending on the bottom.
ケ and ク
In ケ, the cane (ke), the second stroke (representing the arm) extends over the cane. In ク, the cup (ku) handle is smooth.