Here's three mistakes that keep popping up...
1. Information overload
There's hundreds of ways to make money online - and a few dozen with these forums alone. How do you know what to do?
With Adsense, affiliate programs, product creation, seo, ppc, cpa, and a million other acronyms, it's tough to just pick one to work with.
For my first few years I spent (see: wasted) a whole lot of time floundering around with different techniques and made a few dollars here and there, but nothing to write home about.
It wasn't until I stopped spreading my time across multiple avenues that I found a single income stream that's been really profitable for me.
Since then I've been able to diversify as I've been able to automate certain aspects of my income... but until you STOP doing everything and start doing only one thing you're probably not going to make much money.
You'd be surprised how much money someone can make when they get up off their butt and just do something.
I see a lot of people over-analyzing their offers, endlessly critiquing their copy, or just being scared to take action...
Follow the ready-fire-aim approach instead.
Choose something, do it, and tweak it.
If you don't make a lot of money right away with it, keep tweaking it until you do. It's worked for me, and it can work for you too.
3. Not associating with others
IMers, in general, tend to be loners. We like to sit in front of our computers all day every day working.
It wasn't until I started congregating with other IMers (online or otherwise) that I saw a big increase in my online income.
Here's the facts... You need help from other people. I need it every day.
If you can find a group of people to associate with that are at least one level above where you are, they're going to help you make a LOT more money.
Think of one single thing that you can do today to impress someone that's making more money online doing what you want to do, and do it. Get them to notice you and you'll find that they'll quickly be your friend.
So, those are just three of the big things you need to watch out for... What else can you think of?