Make your characters as different as possible, so they stand out as individuals. This includes their appearance, speech patterns, and personalities. For example, give each one their own pet phrases or expressions. If they all say the same one, even if it’s an expletive, it doesn’t distinguish him or her as an individual. Of course there are some expletives that are more popular than others, and thus pervasive, but if you’re original in some way, it adds a touch of creativity which most readers will appreciate.
Unique appearance that is distinctive and reinforces the character’s personality is helpful to the reader in keeping them sorted out as well. Gestures are another way, whether it’s rolling their eyes or dismissing a statement with a wave of their hand. A unique perfume or aftershave fragrance is another way to bring another sensory detail into the story. Pay attention to the people you know the best and what their individual habits are for ideas.