Any small fidgety thing unique to a specific character can, with repetition, be a character tag. Habits like smoking or eating salad with chopsticks indicate things about your character. Visual cues, like special always-present articles of clothing and jewelry or personal grooming (immaculate or otherwise) indicate other things about your character. Mannerisms may indicate nervous tics or military backgrounds; speech habits may indicate regional origin or personal swearing preferences. Watch your friends and acquaintances for personal and especially against-character quirks, like the biker who won't swear in front of children or the teen who adopts an old hat and wears it everywhere. Real life is the best guide for developing great character tags.
The specific words you use when creating character tags help define your character to the reader. For instance, a character who always dresses in black can be chic and sleek, goth and emo, or rather creepy. A character who picks his nose when he thinks no one is watching strikes readers one way; a character who primps every time he sees a mirror seems quite different. Choose tags after you have developed your character, and they will blend seamlessly into your writing.
Certain character tags, if not overused, can be great dialogue indicators if used in place of the traditional "she said." For instance:
"I was just going to work," Irene said.
"I was just going to work." Irene absently twirled one curl around her fingertip.
While the first is acceptable, the second, by using a character tag, not only identifies the speaker but adds to a sense of place and more firmly establishes the speaker's character.
Character tags can also be valuable personality exposition tools. In the example above, Irene may twirl her hair only when she is nervous or lying. If that has been established, the lie is identified to the reader and the tension of a scene ratchets up. In the normal scheme of things, character tags can also identify specific personality traits, like the fictional TV detective Monk's obsession with cleanliness revealing his obsessive-compulsive disorder.
In detective novels, subtle use of character tags can identify the villain and others to a reader. For instance, one character may chain-smoke Pall Mall cigarettes interspersed with clove cigarettes to, he says, cleanse the palate. A scene in which stubs of both are found behind a bush at a murder scene indicates the character at least knows something about the crime.
Writers should seek the Baby Bear balance for character tags: neither too much nor too little, but just enough. Too much character tagging annoys the reader; too little tagging leaves characters with vague mannerisms that don't really identify them. Stick with one or two tags per character, and use them in the scene where the character is first introduced so the reader knows to associate that tag with that character. Use tags sparingly after this, keeping in mind you can always add them later in spots where you need more character color. Beta readers are the best test for too much tagging; when your beta reader complains, it's time to take some of the tags out. Also, never use the same tag for more than one character.