How to use a sunscreen. Advice from Dermatest Australia (leading authority and testing agency)
It is essential to use sunscreen correctly to achieve protection.
• Apply sunscreen liberally to all sun-exposed areas so that it forms a film when initially applied. Most people do not use enough. It takes at least six teaspoons of lotion to cover the sun-exposed areas of the body of one average adult person.
• It takes 20–30 minutes for sunscreen to be absorbed by the skin and it can easily be rubbed off, so apply it at least half an hour before going out in the sun.
• Reapply after half an hour so that the ‘mountains’, as well as the ‘valleys’, are protected (imagine you are painting a wall – two coats of paint provide a more even coverage than one).
• Re-apply sunscreen every 2 hours if staying out in the sun for more than an hour during the day.
• Re-apply immediately after swimming, excessive sweating, or if rubbed off by clothing or towelling, even if the product claims to be “water resistant”.
• Insect repellents reduce the sunscreen's SPF so when using together, use a sunscreen with a higher SPF and re-apply more often.

Keep in mind that while crucial, sunscreen alone is not enough. Seek the shade whenever possible, wear sun-safe clothing, a wide-brimmed hat and UV-blocking sunglasses, for a complete sun protection strategy.