The biggest mistakes of my early Internet Marketing career:

1
Investing a lot of time and effort in creating and promoting a non profitable "free info" website.
Keyword intention is paramount.
I would much rather have 500 visitors for "buy laptops online", than 1000 visitors for "free laptop tips" ....
Now I focus on profitable micro niches. More on that in my last point.

2
Trying to find a new WP theme for each website and learn all the intricacies of each theme. I wasted countless hours in the early days messing around with different themes on different websites.
Learn to prioritize, and be efficient in regards to time and effort as well as money.
To be ultra efficient, find one great theme that you can apply to many websites. I use one theme for all my sites. It's very clean and simple, but still quite stylish and modern.
I don't subscribe to "gurus" who tell you to use default themes. Yes, simplicity is important, but so are first impressions.
There are lots of tasteful themes out there that look simple, but give the impression of being custom designed.
Check out Aqualine, Business Lite, Photoria, Paragrams
...... Pick one.... Learn it backwards...Use it on multiple sites with minor tweaks to logo and CSS.

3
Not knowing how to use Excel.
I only got into Excel about a year ago and it has become one of the most powerful tools at my disposal. I use it for keyword research, niche research, keeping logs, and more.
In internet marketing, maths trumps gut feeling every time.
Doing some simple calculations can be very revealing. If you want to get into Excel but don't know where to start, you could start by setting up a simple spreadsheet which calculates the average PR for the top 5 competing websites for any given keyword. You can then refine the spreadsheet to use more complex formula to give an overall score for any keyword, taking into account different factors.
Want to get real fancy? Get Market Samurai, export some results and use your own formula to weigh up which keywords you can tackle.


4
Being intimidated by keywords with large search volume. I aimed way too low when I first started, and I ended up ranking number one for a handful of keywords with dismal search volume. Unless you're selling yachts or insurance, low SVol words are not worth pushing for.
Now, I do look at those keywords that are getting upwards of 100,000 monthly searches, and often I can tell from a quick glance at the SERPS.. I can rank for that!
Learn to assess keyword competition. There are still lots of profitable key-phrases out there just begging to be optimized, and new ones are being born daily as new products are launched, new hobbies gain popularity, and pop culture throws up new buzzwords.


5
Not seeing the bigger picture.
I'm a graphic designer. When I first started in IM, I figured I could make nice looking sites, and blag my way through other aspects of website creation, such as content, and SEO.
Successful websites are like houses of cards: every single piece of crucial!

Your website needs to:

• look smart, bespoke and trustworthy
• have valuable content
• rank for relevant keywords that convert to money

Find your websites weak points and fix them.
In my experience, newbie marketers tend to have lazy content, and tasteless themes. If that sounds like you, don't be surprised you're making no money.

6
I was badly organized and wasted countless hours sifting through bookmarks menus, and desktop folders.
Set up a well organized Speed Dial on your chosen browser.
It will save you hours if not days in the long run. I recommend SPEED DIAL FOR FIREFOX:
https://addons.mozilla.org/en-US/firefox/addon/speed-dial/

7
Finally, when I first started, I went for really dang obvious niches, such as DVD reviews and digital cameras. (face-palm!) These niches are completely saturated.
Now I find micro niches packed full of enthusiastic hobbyists. If you have the right combination of objective strategy, and creative brainstorming, you CAN find those micro niches that haven't yet been mined.
Try some cross pollination, monitor some trends, get out of your comfort zone, keep your finger on the pulse.