Viable ideas usually come out of a community that's at the forefront of some change in the world, either technical or social. Embed yourself in such a community and then build what you and your friends need, and chances are the rest of the world will eventually come around to seeing they need it too. If you're explicitly trying to brainstorm and evaluate ideas rationally, particularly if you're doing it in isolation, you will a.) bias yourself toward ideas that sound good but aren't what anyone wants b.) bias yourself toward ideas you might've heard of in the press but are already filled with competitors and c.) bias yourself toward ideas with big potential markets but that nobody actually wants.
2. Trying to solve obvious problems (e.g. death, disease, etc.).
Initial "marketing" (word-of-mouth) is key. Will there be a niche to start with?
The levers to pull on are 1) how many people are impacted by the problem, 2) how expensive it is to solve the problem and 3) people's propensity to pay for a solution.
Hailing a cab in SF on a major holiday sucks; enter Uber/Lyft.
Staying in hotels with your family sucked, people wanted to stay in a home; enter AirBnB.
As a sales rep, tracking deals always sucked; enter Salesforce.
Going to the grocery store sucks(a lot of times); enter Instacart
There are no shortage of things that suck, just people willing/able to solve a given problem.
You'll see a problem or a pattern that a new business could provide value to customers to solve.
Definitely check out the microconf videos, linked to in another reply.
The StartUpsForTheRestOfUs podcast is a great resource as well. Rob and Mike run MicroConf. Lots of great advice in their archives.