If it's a "trend", it's too late
I disagree. In fact, if something is a trend, it's a guarantee that a market (of some kind) exists and it can save you a lot of time and research. Most big markets have large numbers of high value competitors and they're not all pioneers. Indeed, most companies with significant turnover were not pioneers in their field - they came in with "me too" products that merely improved upon the existing popular ones before they became too entrenched.
The trick is to get in when something becomes a trend but before the masses realize it's a trend and you have hundreds of competitors making similar improvements to the core products and services as quickly as you are. As well as being my belief, it's one of the core tenets of Michael Masterson's tackily-named (but a great read nonetheless) "Ready, Fire, Aim: Zero to $100 Million in No Time Flat."
With your reference to the App Store, I suspect you might agree with this, but have a different idea of what a "trend" is. I consider the App Store and developing iPhone apps to have been a trend in late 2008, not long after the earliest developers were starting to share revenue/sales figures, but before the masses had figured out it was going to be a giant cashcow. Now developing iPhone apps is just "what you do" rather than a trend, IMHO.
The good thing is what you said: market exists. There's money there, just make sure it isn't all taken up already.
The market very often has been there quite a while with major players in it that are concentrating on monetizing their current offerings while being completely blind to the fact that new possibilities are now available. This tends to happen to startups that have "matured", generally replaced all their leadership and now have very little internal vision.
If you can spot one of these opportunities it can be a true goldmine.
They take over oil fields after the big oil giants want to abandon the well and usually manage to squeeze more out of the well in a more efficient and environmentally-friendly manner. Their profits are generated primarily in Africa, and they also work better with local officials than the big players do.
Maybe BP should contact them and hand over the Gulf mess to them :)
I've been trying to get far more disciplined in looking for the more practical, boring problem/solutions that follow market trends etc.
I just find it so hard to do because it doesn't excite me as much, even though I know there is so much opportunity out there.
I'm pretty deep in the Ruby world. A trend that has emerged (and is almost becoming a cliché) over the past 18 months or so is selling Ruby related screencasts. Peepcode started selling screencasts about 3 years ago but no-one followed on in a serious fashion for quite a while (until Peepcode "proved" it could work, I say).
Now, there are at least 7 - 8 unique vendors of Ruby related screencasts I can think of. None are Peepcode scale at all, but I've seen sales figures and know that some of them, at least, have done reasonably well. Even the people behind the more humble attempts are happy, see: http://codeulate.com/2010/03/how-to-sell-a-hundred-screencas...
Now, "Ruby screencasts" is a crazily tiny niche. Not only is "programming" a tiny niche in the world of business, "Ruby programming" is a MINISCULE niche in the world of programming.. and Ruby programming screencasts is ULTRA CRAZY MOLECULAR LEVEL TINY!
If people have picked up on a trend in such a miniscule market and are making money with it, that's says, to me, that there are millions of teeny tiny niches in which you can start to perform very simple "tests" that make money and which, ultimately, could lead you to that million dollar concept or market.
Picking up on these teeny, tiny "trends" can be tricky, but is a lot easier in fields you're already absorbed in. If I had to pick out some trends just from reading Hacker News recently, these seem like a few areas someone could make some bucks in now/soonish: node.js consultancy/screencasts/books, Posterous theming, Flash-to-HTML5 tools/consultancy/screencasts/books, non-college oriented compsci education.
I first got my starts by trying to blog about video games. Then I started a wiki which I bootstrapped off my own saving. Now it produce maybe 20-30 bucks each year. Not a whole lot, but I bootstrap my way with that money in addition to programming work that I started working.
I am hoping to go back in automating the maintenance of my wiki and create new sites in that niche to generate a bigger income, which then will be used to bootstrap other sites.
The way I figure it is that for each domain, I only need to make about 15 bucks to be profitable. Basically, they serve as buffer money used to fund new mini-ventures.
It may seem boring at first, but let the chase of winning excite you, along with the money. Odds are you get bored because you're not interacting with customers and/or making money from it. I've fallen into this trap before. Seeing money come into your bank account or have customers contact you will get you insanely pumped. It's not about the money in a greed sense, but in the "i've done something people find valuable sense".