5) Save. Loads.
This is obvious, really. Mistakes can be fixed, things can be tweaked or rebuilt but if a level crashes and you haven't saved for hours then you'll feel pain like you've never felt before. So save all the time. After every change if you can.
To be extra safe copy a level to different patches on your moon occasionally so if something catastrophic happens you have an unsullied back-up to return to. You can also publish locked, unplayable levels to store them online. So even if your PS3 catches fire and melts, your work will be safe.
6) Never use copyright material
Whatever you may think about copying other people's stuff, the fact remains that “copyright” means literally “the right to copy”. If you don't have that right and use Watchman pictures or Metroid game art to decorate a level, it could get pulled. You can pout all you want but if it comes down to a shouting match between you and Warner Brothers' legal department over your Dark Knight homage featuring images from the movie, guess who'll win.
Direct copying of other people's material just isn't worth the risk. And anyway, why aren't you creating your own ideas? If you must build a Metal Gear Solid tribute then make it 'in the style of' – if character design, layout and names are sufficiently different you might get away with it. However, it all depends how seriously whoever you're ripping off takes it. Be warned.
7) Play other people's levels
What? You think you're the only one with ideas? All those other creations out there aren't just for show, they're a valuable resource packed with inspiration. It could be a clever idea for switches or an ingenious way to use checkpoints. Whatever it is you'll learn loads by seeing what other people are doing. What makes one level boring and another fun? What makes you stop playing? Or carry on? Ask these questions as you play other gamer's work and you'll be an LBP Don in no time.
8) Use the grid
Dip into the pause menu and you'll see the option to choose a small, medium or large grid. This provides a graph paper-like backing that the cursor will snap to. The most important one is the large grid which is built around the distance a sackboy can comfortably jump. Vertically it's one square up and horizontally it's three squares across. If you use that as a guide you should build a well balanced, fun platformer.
You can use any grid, however, to construct things precisely and in grid mode separate blocks can be created touching each other without joining together like they would when free-building. For extra precision you can use Front View which is also in the pause menu. It flattens the perspective to 2D, making it easier to line things up. Use Game View to switch things back to normal.
9) Test and test again
Just like point 3 (Tell people what to do) when you've been working on your game for months it's easier to loose perspective. You might make a certain jump every time because you've done it a hundred times. Of course your level is easy for you, you built it. But to someone playing it for the first time it could be an insurmountable challenge requiring nerves of steel and pixel perfect timing.
Poor gameplay sucks the fun out the room like world politics which is why real developers spend a fortune on testing and tuning their work to make sure it's fun. So get your friends involved - ask anyone you can find to have a play and see what happens. The more you test it the better it'll be.