Scalable revenue-generating assets.
Let's work backwards on that:
Asset: We're going to call an asset as something that can generate value or appreciate in value. So your 2005 Honda Civic is not an asset, but a 1969 Corvette Stingray might be. Websites can be assets, so can intellectual property, contracts, royalties, real estate, stocks, etc. So step 1 - you need asset. Things that can go up in value, or generate value.
Revenue-generating: You can buy for appreciation, but that takes a lot more money and skill. So you want revenue-generating assets to get passive income: That means things that kick out money regularly. That would be real estate where the rents are higher than the PITI (principle, interest, taxes, and insurance) + maintenance. That could be a website that you can make money from - ad revenues, product sales, etc. It could be something you've written that you get royalties from.
Scalable: And here's the magic that puts it all together - scalable means you can build it bigger without it demanding more of your time/resources (like a job does). Real estate with positive cashflow tends to be scalable, because if the property appreciates even 10%, traditionally you can mortgage a new property. If you don't mind doing repairs/adding value, you can mortgage a new place, repair/upgrade it to get higher rents, do so, have it appreciate, use the new equity to get a new place with cashflow, etc. Some high percentage of self-made millionaires in the USA are from real estate due to this fact.
There's other ways to get scalability. Obviously a useful website, or something with automated fulfillment or marketing. But scalability is key: All the time, if you want passive income, you need to spend money out of your profits to remove time and automate more. So pay a fulfillment company $2.50 per package (on top of the postage) to mail your thingy for you. Get into automated marketing with Adwords or Adsense. Look to ink deals that'll keep paying you without you putting in more effort.
The formula's Scalable Revenue-generating Assets. That's how to build gradual increasing passive income. It's harder than it sounds (like everything worthwhile is), but completely doable if that's what your goals are. Just don't forget that active income is pretty cool if you love what you do.