I came up with 110 ideas in the last 12 months for startups or innovations (I record and evaluate and categorize them using trello, and decided to count then the other day).
I didn't start becoming prolific with idea generation until after I adopted a system where I could easily record ideas and categorize them and easily evaluate them. I think having a system to do that is perhaps the most critical component becoming prolific enough of the process that you end up with a sure-thing startup idea.
Similar to most things, the way you get good at generating ideas is by practicing generating ideas (and recording them and evaluating them). Develop a system to do so quickly and easily (kanban systems like trello are pretty good at that). No matter how small, if it's an improvement, or a problem needing improvement, record it.
I've also discovered that some of my best innovations have come by tangents to the original innovation. Pivoting to the new-and-better ideas gets harder with the more players (especially investors) that are involved, which is why I believe most shouldn't do a startup until they've become good at generating and evaluating ideas, with a bucket full from which the best has risen to the top.
So it's an ability that you should develop during your day job if you can. If you're ready to do your first startup now and are looking to generate that sure-thing idea and don't have one yet, then join someone else's startup while getting good at idea generation, recording, back-of-envelope evaluation (risk / reward analysis, market fit, playing devil's advocate), etc. Once you get good at generating ideas of you haven't learned to quickly order them from best to worst, you've really not gained much, so that's also a skill you should acquire before thowing all your eggs in one basket.
Also keep in mind 80% of my ideas are best as pitch-and-sell ideas (inventions, process improvements, etc), not startups, and I think that is often the case. Those are also great, as they're easier to do, and have a lot of overlap with startups. When doing that focus on volume, generate and share sell sheets, and be dispassionate enough to keep the ideas still flowing. Check out Stephen Keys YouTube's for ideas how to crank through those while cogitating your ultimate startup idea.
Being the cathartic genesis for your startup is not necessarily the most sure way to startup siccess, but it will give you passion to really make your startup succeed... A passion that goes beyond the desire for financial success. That will be a driving influence for you and for those whom you are able to convert to your vision, including team members and investors.