Here are a few tips to help you.
• If you grab a Mega Mushroom while using a Gold Flower, the Gold Flower will be stored in your item holder until you use it again.
• Gold Flowers will double the value of golden enemies, golden mushrooms, and Star Coins.
• W 5-A is a great place to get alot of coins without sacrificing time, because it is an auto-scroll stage. Just use your Gold Flower, stomp on the first Koopa you see, and then throw it onto the ship. Get every Koopa in the level to get hit by that shell after you jump through one of the level's many Gold Rings. Eventually, you will get 100 coins per Koopa.
• Touching the very top of the flagpole will double your amount of coins.
• Collecting Star Coins will net you 10, 20, and 50 coins, respectively.
• With some auto scroll stages, timing is key.
• You can destroy almost every enemy in the game with the Gold Flower, including Thwomps, Podobos, Spike Balls, and Dry Piranha Plants.
• Gold Brick Helmets will always contain 100 coins.
I hope these hints will be helpful. Happy collecting, everyone!