It is important to compliment people around us in the modern world. If you go for a managerial position in your workplace, learning good people skills becomes essential. If you have a crush on some woman who you wish to go out with, you must learn how to chat up with her and make her happy. If you are in school, being your professor's pet may help you get better grades. Then, who says that giving compliments to people does not matter?
We all have the capability to say positive things to people but we just rarely do it. From now on, try praising your employees if you are a supervisor or something and it will make them more productive and satisfied in their jobs. Few words like "good job!", "You have done so well this time", for sure, they will actually try harder.
Give a girl a sincere compliment without making them feel like you have a ulterior motive. Put a smile on her face, and don't just think about making a play on her, because no one likes a phony guy. Doing so will definitely give her a high for the day. Also, Do consider where the cut-off point is. Avoid doing anything to them like borderline sexual harassment while complimenting ladies around you.
How do you know if your compliments work? Well a simple way to tell is, if the girl accept it, she may respond to you with a compliment as well. Then you you are on the right track!