+ Be Comfortable

Experiment with how you position your arms to figure out what feels good for you (arms over-head with palms facing up, arms at the sides with palms facing down or up, arms across your chest, arms across your stomach).
Relax your muscles starting from your feet and moving up to your head.
Tilt your chin up so that the back of your head is floating and the water is framing your face. This should alleviate any tension in your neck.

+ Be Still

Try to remain still and not generate momentum while you're in the float room.
If you touch a wall, gently push off so that you don't bounce off from one wall to the other.
The quicker the water settles, the quicker your mind settles.

Let go of any expectations you have for your float - expecting to see something or feel a certain way during your float can be distracting and frustrating.
Clear your mind - try focusing on something (your heartbeat, your breath, etc) and when you catch your mind beginning to wander, bring your focus back.
Let go of the concept of time - if you find yourself worrying about how much time you have left, distract yourself by focusing on your breath or counting back from 10.

Focus on taking deep breaths while you're settling into your float - breathing is a great relaxation tool that can help you unwind.
Breathe in and out through your nose.
Observe your breath - your breath will become shallow as you become more relaxed.

Be conscious of how your arms, hands, legs and feet feel - with no external stimuli coming in to distract you, your senses will be heightened.
Take this time to observe any thoughts/emotions you experience.

Articles on "Tips for Floating"