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Like most other parts of your body, you can actually train your lips to become stronger. One of the best ways to do this is by whistling. Whistling puts pressure on your lips, and encourages blood flow. Try whistling whenever you've got a moment to spare. As well as making for a pleasant musical distraction, casual whistling will help you get fuller lips in the long run. Puckering is the go-to exercise for building up strength and lustre in your lips. Pucker up your lips as though you were about to kiss someone, then press your hand up against your lips to meet that kiss. Withdraw your hand after a few seconds, and release your pucker while exhaling. Following these steps count as one 'rep' of the exercise. Repeat as many times as you desire in a session; between 10-20 should be fine to start. Although such frequent puckering may feel strange at first, you'll find your lips getting stronger after a few sessions of this exercise. Breathing exercises can also help plump your lips, provided your lips are given some strain in the act. Pucker up your lips, inhale air through your nose, and exhale while holding your pucker. This will stimulate your lips and help in giving them a plumper look. Between 5-10 reps should make a good workout session for your lips. Repeating these sessions daily, or bi-daily will ensure the appearance of more lustrous lips. For a quick-fix for bigger lips, there are many simple lip pumps and 'cups' that plump your lips via suction. Placing a lip cup on your mouth and creating suction will create pressure and blood flow, giving a temporary boost to the size of your lips. Many of these cups are quite cheap and easy to use, and they're suitable for bringing with you to social events, to use when you need some freshening up.

Summary:
Whistle! Exercise your lips with puckering. Try breathing exercises to work out your lips. Use a lip pump.