UFO Bonus
Every so often a UFO will fly on the screen, when it does tap on the popcorn icon in the right corner of the screen in order to activate the 2 1/2 minutes worth of double speed. If you then, tap the UFO you will have a chance to get the 5x earnings for a span of 50 seconds which is the equivalent of one straight minute where you have 10x your normal income. Make sure ALL of your planes are out on the track when you have this bonus.
Free Offers
Whenever you sign back in to the game after being away for a while make sure you check the store to see if there are any free offers available which you can take advantage of. There is a always the possibility that you will get a free plane upgrade which will give you the opportunity to get a 3-4 levels upgrade.
Hexagon Gold Coins
This special currency can only be obtained by trading standard coins for them. They are not cheap as the ratio is 10,000 to 1 but in the long run you will find the investment worth it as you will be able to purchase plenty of permanent boosts such as income multipliers.
Air Controller
Make sure you pay attention to the air controller that is in the bottom left corner as he he will periodically offer you quests in game that have achievements that will give you the opportunity to keep your gems topped up.
Speed Upgrades
Resist the temptation to spend the gems you earn on new planes. It is much more beneficial if they are spent on speed upgrades or spins on the wheel. Although spins on the wheel is expensive they are worth it to get the coin increases they dish out.
Keep Planes Running
Make sure you don't neglect your track while you are buying new planes and combining them together. It is important that you keep your higher-ranked planes running on the track with some also on the parking apron.