YC once tried an experiment of funding seemingly good founders with no ideas. Every company in the no-idea track failed. Good founders have lots of ideas about everything, so if you can't get an idea for a company, work on getting good at idea generation first.

Surround yourself with people who have a good feel for the future, entertain improbable plans, are optimistic, are creatively smart, and have very high idea flux. The best ideas are fragile—most people don't even start talking about them because they sound silly. You want to be around people who don't make you feel stupid for mentioning a bad idea.

Stay away from world-weary people who belittle your ambitions. Unfortunately this is most of the world. They hold on to the past, and you want to live in the future.

Project yourself 20 years into the future and think backwards. Or identify the biggest tectonic shifts happening now—in such shifts the world changes so fast that big incumbents get beaten by fast-moving focused startups. The mobile phone explosion of 2008–2012 was the last big one; we are overdue for another.

Differentiate real trends from fake ones: a real new platform is used a lot by a small number of people, not a little by a lot of people.

Any time you can think of something possible this year that wasn't possible last year, pay attention—especially if next year would be too late. When you can say 'I'm sure this is going to happen, I'm just not sure if we'll be the ones to do it,' that's a good sign (Uber felt that way to me after the first ride).

Ask early: 'could this be huge if it worked?' Most businesses don't generate an accumulating advantage as they scale. Think early about why an idea might have that property.

Founder/company fit is as important as product/market fit. A good final test for an idea: can you articulate why most people think it's a bad idea, but understand what makes it good?