New Super Mario Bros. U is not that hard if your only goal is to bring down Bowser and his band of pesky kids. If you want to get all of the Star Coins, on the other hand, your task immediately becomes a whole lot tougher - especially if you decide to find them all on your own. Doing so is more satisfying than relying on a guide, but it's quite difficult.
Guided or not, the Star Coins are difficult to collect. This guide, therefore, will provide some general hints on gathering the hundreds of Star Coins littered throughout New Super Mario Bros. U's levels. Pay heed - these lessons were learned through many painful hours of searching and dying.
Rule #1: Search everywhere
If you heed no other rule in this article, at least listen to this one. Some Star Coins are sitting in obvious places; most aren't. Indeed, the vast majority of Star Coins require you to explore out-of-the-way spots and hidden niches that have nothing to do with progressing through the levels. Check every wall, every (safe) gap, every pipe and door until you find all three coins in a level.
Rule #2: The Flying Squirrel Suit is your friend
Nine times out of ten, a Star Coin will be easier to get if you have the Flying Squirrel Suit. There are a few exceptions - primarily when you have to kill an enemy with Fire Balls or create an icy platform, for example - but for the most part you're best off keeping a Super Acorn or two in reserve. Learn how to boost jump effectively with ZR to turn tricky Star Coin snags into a breeze.
Rule #3: There are other ways to gather Star Coins
You don't have to touch a Star Coin to collect it. Instead, you can throw an enemy at the coin (usually a Koopa Troopa shell or iced foe you can pick up) to get it instead. This method is an easy way to snag coins that would require hairy jumping or gliding. You can usually tell if a Star Coin is meant to be gathered by tossing an enemy if said enemies regenerate after you leave the screen and come back.
You also want to pay heed to any POW blocks you find. Hit a POW block where you've found it and you'll almost always bring a Star Coin raining down on your head. Make note of the terrain, however, as these coins often fall into areas that are unstable. Move quickly to snag your Star Coin before it slides into a pit.
Rule #4: Fake walls are everywhere
More than many past Mario games, New Super Mario Bros. U is absolutely teeming with fake walls and secret passages. They're to the left, to the right, above and even sometimes below you, and you must explore to find them. Run into walls and leap against ceilings to test for gaps. You can usually spot fake walls thanks to divots or gaps in otherwise straight geography, but not always.
Similar to hidden paths are hidden coin blocks. Many Star Coins that look tantalizingly out of reach are sitting under invisible blocks. Jump around a bit and see if you can find these potential stepping-stones.
Rule #5: You don't have to go far to get a Star Coin
The path to a Star Coin is typically within a screen or two of spotting said coin. If you see one and you can't get to it where you are, you seldom ever have to travel far to find the path. Check everything in the nearby area for the way to the coin.
Rule #6: Try, try again
Unlike some past Mario instalments, the Star Coins of New Super Mario Bros. U you collect are recorded each time you complete a level. Consequently, if you get two out of three coins in a level and then beat that level, you won't have to collect those two coins again on your next play-through. You can use this fact to your advantage by collecting particularly tricky coins, then beating the level and going back to the easier coins. Some Star Coins are so hard to get that you don't want to repeat the experience twice.
Along these same lines, any Star Coins you collect prior to the midway flag in a level are saved. If you want to get all of the Star Coins of a level in one go, snag the easy coins, hit the midway flag to save, then go for the toughest coin. If you die you'll still have the initial coins you collected.
Rule #7: Stockpile lives
Many Star Coins are floating over endless chasms or pits full of enemies, or are otherwise found in rather perilous locations. You will die while looking for Star Coins, and chances are good that you will die a lot. Offset this risk by hoarding dozens of extra lives that you can burn through whenever you hit a particularly troublesome Star Coin. The best places to get extra lives are the 1-Up Mushroom Houses littered throughout the game, though the Swinging Ghost House in Frosted Glacier is also good, as it's teeming with coins.
Having trouble with lives? Try beating the level first. After you've beaten any of the levels, you can hit Start and choose to Quit the course from the menu. Do this before Mario dies and you won't waste a life - and if you've already hit the halfway flag, when you restart the level you'll be back at the flag.
Optional: Try a guide
Try as you might, you'll have a hard time finding absolutely every Star Coin on your own. Some are fiendishly well-hidden, or require little tricks that aren't immediately obvious. If you're having trouble, check out the guides below for hints on finding all of the Star Coins in New Super Mario Bros. U (though Superstar Road is still pending).