If you are unsure on how to do the storyline for your game, a good easy way is using a common comic styled effect. This allows you to tell the story well in a way players can read quickly. It works well in short games. It's important to try add your own style to it though so it's not too much like all the others. A picture below shows them using it in this game Freeway Fury 2 on Kongregate.
A while ago as practice for fun I had a go at recreating mario, the physics still arn't perfect, but I think I got them pretty close. Was fun programming an old classic. This is a good way to practice making platformers smooth and seeing how mario did it. Play and/or download the engine below.
Click below to play the engine. Arrow keys to move.
Click below to download the flash file
It's in Actionscript 2.
Something to keep in mind when making flash games or even any other indie games, is that they are going to be smaller than a proper console/pc game wether you like it or not. So instead of fighting the urge to expand on everything, sometimes it will make more impact with small cute characters and graphics. Sometimes smaller is better if the game is small. Theres no point having over done realistic graphics for a simple cute platformer. It's overkill. So don't stress out and try aim for smaller cuter things when starting out and leave the realistic tough stuff for when you tackle a big game.
Not all games you make are going to be a success unless you are simply perfect and no one is. One example of this for me is Battle Egg. The game was one of my worse flops after release. It didn't get sponsored for any money and made only $100 in ads over a year or so. When you work on a game for a long time it's important to step back and try look at it from a different point of view. Will others like what you just made? or is it only fun because YOU made it. This is a hard thing to get past, everything you do will always seem better to you then others because you know all the work behind it, you get emotionally attached so to speak, so it's best to try take a step back from that.
After a few months when you've cleared your head of the making of the game it's easier to judge. I made the art a little too messy and un-polished. The upgrade screen was way too simple and needed more upgrades and the UI was not nice to look at. The gameplay and idea I still thought was good but I badly executed it. The game was also too short. These mistakes are good reminders though to look at and learn from. I learn more from my mistakes more than I do from success usually. Designing is a tricky subject because you may think you are being unique and adding a ground breaking mechanic only to find most people don't like it even if it is new.
Overall don't get down about bad projects you release, just think of them as learning hurdles to get past.