Will it happen instantly, or will the government still try to withhold evidence? For a full disclosure about UFO's to happen one or more of the following events will have to be triggered; The Disclosure Project.
1. Photographic Evidence
A picture is worth a thousand words. This statement rings true when we are talking UFO sightings. With the advent of cameras in cell phones and the quality of pictures they take it will only be a matter of time before, multiple pictures are taken of the same object from different angles. The more angles of photographic evidence the harder it would be to dispute. Today, these telephone cameras produce video images as well, even better than stills. The chance of a fake picture passing scrutiny (Photoshopped pictures) is almost nil to none existent, even though Photoshop pictures look real they can be detected.
2. Investigative Journalism
It's amazing UFO sightings haven't been taken seriously. Perhaps a journalist somewhere will take a stand against being hushed by the government. It's easier to ridicule a subject like UFO's than to investigate them. Opinion in these matters seem to pull more weight that evidence. Reporters have to break away from the norm and leap into the unknown.
Someone from WikiLeaks or an organization similar happens to want to tell their story about a government cover-up. Someone with inside info, that can back up accusations with facts.
4. Deathbed Confession or a Public Statement
Someone in charge, the President, the head of the CIA, the many astronauts that have hidden evidence and knowledge from the public decides to tell the world. Could you imagine a person like the President of the USA on his death bed confessing that UFO's are real? You'd have to believe him. Unfortunately, over the years many people in high places have confessed to strange aerial phenomena on their death-beds to no avail. Their confessions certainly could have been a start for Disclosure but fizzled out and were stifled by the higher-ups as soon as they became known.
5. Physical Evidence
Nothing speaks more than physical evidence. We need a body, a piece of equipment, a piece of the UFO, actual parts witnessed and documented by more than one or two people. Why haven't these implants people have taken out from under their skin been investigated better? Do they still not know what they are?
6. Declassification and Discussion
The government opens its files and discloses to us what they know about UFOs, with no blackened out pages and paragraphs.
7. A Mass Sighting
We've had mass sightings before. Remember the Phoenix Lights, sightings over Phoenix Arizona? The more the merrier I always say. The Phoenix Lights did happen, we have photographic and eye witness accounts to prove it. This would be a good precursor to getting the public involved.
8. The Aliens Make Themselves Known
No explanation and the worst senerio. If this happens, it will be too late for a cover-up. The world will end as we know it.
All will come to the realization we are not alone, and hopefully come to earth in peace. Just the fact they show their faces will be enough to cause major panic around the world. Hopefully they're good-looking, that might ease the fear but from what evidence (real or not) I've seen, that doesn't seem likely.
Any or all of these events or a combination of some of them is all that will be needed for the Disclosure Project to start a chain reaction to "A Full Disclosure."
Further Reading: A.D. After Disclosure - Richard M. Dolan and Bryce Zabel