My advice is...
Just keep practicing flash and watching more stuff here on Newgrounds. And DON'T try to submit things in the meantime. If you try to rush it, you'll submit crap. If you try to submit things without ideas, you'll either fill your account with blams or with crap. Then when you get an idea that may be decent, people will see your account has lots of low scoring bits and pieces, and they'll think you don't care. Stay away from Sonic/Mario/Dragon Ball Z/Madness/hentai slideshow -type ideas for flashes. They're over used, over done, quick-fix ideas that will bring in low, but passing scores. They're easy points, but cheap points, and they'll bring the quality of your portfolio down.
So for now, watch and play stuff on Newgrounds. Don't stick to the Top 50 or the stuff that was just submitted. Search out things with scores above 3.50, and things on the pages that are further back than the main page. Take a run thru my favorites. There's good stuff there. Fill your mind with memories.
Eventually, one memory will build on another memory and you'll come up with an original story or idea. Also, listen to music. Not just rock or hip hop or metal or whatever you normally listen to, but mix it up a bit. Eventually, you might hear a song and an idea will pop into your head as you listen. Many songs tell stories that can be animated. Even songs with no words. Ever see the movie Fantasia? All music, no words, great stories.
When you get an idea, PUT MAJOR TIME INTO IT! If you can get it done in a day, you're not done. If it takes you a week, you're probably still not done. I've seen many good ideas that passed but have horribly low scores because they got an idea, belted out a quick animation, submitted it, and now they're stuck with a low score on a crappy flash that was originally a great idea and could've been awesome, but they ruined it by rushing. Taking extra time to take it from good to great will ALWAYS pay off. Not one person who has ever had something in the top 50 got it there by submitting something that wasn't 100% finished and was EXACTLY how they wanted it to be before they submitted it. If you see things that can be made better, make them better! Work on them until they're the best they can be.