First and foremost: don´t panic! We've all been rookies and we've all done stuff that we now feel a bit ashamed of. Seriously, nobody is THAT cool. Nonetheless, it is important for us to start doing things better, so lets get down to business, shall we?
1. Shooting your camera like its a freaking machine gun: Just because digital cameras allow us to shoot like crazy, that doesn't mean we should. Shoot film now and then, it will help you be more careful and think before shooting something. If you don´t have a film camera, using your digital as one is a good exercise: turn off your screen display, fix your ISO and give yourself a limited amount of shots. For example imagine you only have 24 shots, ISO 400 and try to stick to that.
2. Shooting in automatic: I had a teacher who always said: "To me auto=wrong" Sure, auto works just fine for doing snapshots of your family vacations, but if you want to get serious you can't let your camera call the shots, because well... cameras don´t think. Shooting manual is a must, and soon enough photography math will become second language to you.
3. HDRing the shit out of your photos: Just don´t. If you are going to do HDR, it should be so subtle people wont even notice it. Again, don´t let computers do the thinking for you, learn how to do it in post production instead of using software that will do it for you.
4. Putting huge watermarks on your work: It´s tacky dude, very tacky. I´m against watermarks in general, but IF you totally feel you must use one, put a small one that wont ruin your image. And if you're putting a watermark on a photo you should remember you're basically saying: "this is my work and I want you to see it" so be careful to select excellent pictures to use it on.
5. Using your fan page as a personal facebook: Every time I see another "Someone photography" fan page on facebook I cringe a little bit. You should know that it doesn't make you look professional at all, and if you're going for "John Doe Photography" as your fan page title then you should be putting out AMAZING work or people will feel you have no idea what you're doing. So you must edit very tight instead of posting every picture you shot on your last trip or all the work you discarded when putting up your website. You have to kill your darlings.
I hope this helps you. Its not rocket science its just about being a little more careful on how you're working and how you're displaying your work. Just set yourself apart from the pack and take yourself seriously so others can do it as well.