I have to agree with Opti. Being afraid or too lazy to try new things can be one of the dumbest mistakes. A few years ago I kept getting calls from "some pesky sales person" according to my sec. and I just kept blowing them off for something like 6 months. Finally I accidently wound up talking to the sales person. They asked me to let them sign us up to try this thing, 'it's free, and you can try it on as few or as many of your pages as you want and if it makes you money, fine, if not it only takes seconds to remove'. We'd tried similar things and they were a waste of time. Even after I let her sign us up it was a month or two before I got around to actually testing it out finally when I got really bored one Friday afternoon. Loaded the code onto 4 or 5 otherwise unproductive pages and wound up making $67 the very first day. A few more pages and we were shortly up to $100/day. Devoted the next couple months of my time almost exclusively to studying and refining it and multiplied that figure times over. And, well the rest is history. I still think to this day about how much more we COULD have made if I'd tried it 6 months earlier. Of course I'm talking about Adsense, and I still send that particular "pesky" salesperson a very nice Xmas gift every year and thank her for being so persistent.
Oh, another big mistake I made was choosing our former webserver host and staying with them as long as we did, but that's another long story. We pay three times as much now for better hardware and auto-monitoring, but considering how much Adsense income and search engine ranking we lost every time the old one crashed and noone noticed for hours or occassionally even days, because we were out of town without a laptop or a connection. There is no question picking a bad hosting company is a big money losing mistake.
Oh yeah, and ALWAYS make sure to match up your comment start and end flags!