The bottom lip should curve around the bottom teeth. Make sure that the bottom lip stretches tightly across the bottom teeth. It helps to create a grin and stretch the corner of your lips outwards towards your ears. The upper lip should be tight against the teeth and make contact with the bottom lip. This will help to create the lip vibrations necessary to whistle effectively.
The tongue should be placed in the center of the mouth and connect barely with the front teeth. Think of the tongue as a ramp for the air to travel along. If the ramp is too wide, the air will not travel in a short and fast line. The goal is to focus your air directly over your bottom lip. Make sure the tongue doesn't hit the bottom of the mouth, it should hover just slightly.
The airflow needs to be fast and narrowly focused to create a proper whistle. To create proper airflow ensure that you are breathing and expelling the air with the diaphragm and not the chest. The chest is weaker than muscles in your diaphragm, so don't create enough force to effectively whistle. When you blow, make sure you are tensing the muscles in your abdomen to help expel the air quickly.
It is also possible to create a whistle by puckering your lips. This is a less powerful type of whistle, but it makes it possible to whistle melodies. To do this, pucker your lips as if you were about to kiss someone. Curl your tongue back, out of the way, and then let out a consistent stream of air. This will help you to create a more delicate whistle with more control over the pitch.