1. Make a Safe Choice - Choose a tour that advertises that they are certified as a FAA Part 135 Air Carrier, which means that the operator is subject to higher standards than general aviation operators.
It might also be helpful to look up accidents from the National Transportation Safety Board. You can use this query form to search the accidents database. To get Hawaii results, be sure to select Hawaii as the state and Helicopter as the category of aircraft. When the results open, make sure to open each list to determine which tour company was involved in the accident. I hope this doesn’t scare you off, but it is important to know that there are risks in riding a helicopter.
2. Know the Seating Arrangement - Most helicopters used for touring have six passengers with a seating arrangement of two passengers in front and four in the back. That means that two people are going to be stuck in the middle seats in the back and with limited view. A-Star and Eco-Star type helicopters carry six passengers. The Eco-Stars are nicer than the A-Stars because they offer a little more room, less noise, and more window space.
One helicopter that I particularly liked is the Hughes 500 which seats two passengers in the back and two in the front. I flew with Inter-Island Helicopters on Kauai in a Hughes 500 and it was tremendous.
When booking, request the front seats as they are the best, but bear in mind that seating will be based on the weight of you and your friend/partner. If you are taking a helicopter tour in Kauai, request the right side because most tours circle the island clockwise and you will be able to get the best view of the Na Pali Coast from the right side.
3. Shop online for pricing. Most companies offer a big discount for booking directly with them.
4. Timing your Tour. If possible, try to book your tour on the first or second day of your trip so that you will gain a better orientation of the island. In my opinion, late morning or early afternoon flights are best for limiting shadows and making the island’s colors more brilliant. For seeing the Na Pali Coast on Kauai, the afternoon light is best because the Na Pali is on the west side.