Comment on the environment or situation that you find yourself in.  There is always something to observe and this will seem natural.

Show that you are friendly by smiling.

Hesitation shows a lack of confidence, a major turn-off, so do not hesitate.  Approach her quickly.

Posture matters.  Show your confidence by projecting positive body language: stand up straight and do not look at the ground.

Do not rush at her like you plan to attack her.  This is not football or hunting.  Approach her at a calm pace.

Maintain eye contact as you approach.  If you look away, it signals that you are nervous or unsure about approaching her.

Pay attention to what she says and respond appropriately, while maintaining eye contact.  Looking at other parts of her body is insulting as well as responding to a question without a clue what she said.

Avoid showing your discomfort by avoiding fidgeting.

Use a light tone of voice to begin the conversation.  Opening with a playful joke is generally effective, but pick up lines are cheesy. 

Do not crowd her.  By maintaining a bit of distance, you show that you respect her space and are comfortable with yourself.