I've been writing a book, a book about the UN creating an organization responsible with both defending people agianst malevolent forms of life, and covering up that battle. I'm nearly 20,000 words into it and I call it the Quiet War. I had made a list a year earlier of what types of evidince it would take to send the world into a panic, and what could be done to refute, reduce, or eliminate that proof. I put serious thought into this and if I were part of the organization hiding this, this is what I would do:

First is a list of proof, and second is a list of things to do to compete agianst that proof:

UFO/Alien footage-credibility low
Alien Technology (Must have physical piece, functioning will increase credibility)-Credibility moderate
Alien Corpse (Must show to media, not in video form: that would be classified under footage)-credibility high
Live alien (Must show to media, agian not in video form)-total crediblity

Buy-off-not questionable and costly
Confiscate-not questionable and cheap
Destory evidence-slightly questionable and variable in cost
Destroy integrity (Cause person with proof to look as though he/she is insane, attention hungry, or looking to make a buck)-fairly questionable and low cost
Destory validility-(Done by anti-proof: false but plausable information intended to create confusion)-questionable and variable cost
Threats (Of body, reputation, jail-time, or friends/family)-Highly questionable and moderate cost
Neutralization (Either removal from social interactions, or death)- Extremely questionable and low cost

While these ideas are from a fiction writing perspective, it opens the doors to what people are capable of doing to hide actual proof of what may actually be going on. Also important is who would do this under what lie: for my book, I took the fear of terrorists a good reason to form a special unit within the UN to deal with extremely dangerous terrorists. It opened up a good budget, Electronic survalence of many kinds, and a degree of freedom above the law. Agian, this is in my book, but could be used to hide actual offices intended to deal with cover-ups.

For both my personal interests in the subject of cover-ups and for the sake of my book, post here about what you would like to see or would believe would be done to cover up UFOs. I'll post a small section of the reading when I get around to it. Either a tatical situation (every body loves combat) or a section of text regarding on how I had the characters hide their work.

Personally, I think I live less paranoid not believing that I can be abducted at any momement and tested on by technalogically advanced beings.

I am sure that everything on your list and then some, are actually being done by our government and many others.

I think the biggest, cheapest, and most broad-spectrum tactic would be the disinformation one. Assign a few good agents to spread disinfo, to recruit others to do so, to trick others to do so, and to do everything possible as a full-time job to continuously propagate that disinfo.

do what the media does today,especially on the news,report it at the end of the news and make anyone who beleives look goofy or stupid.

HIDE THE TRUTH IN PLAIN SIGHT!
If you put the truth in plain sight then who will believe that it is the truth?
The majority of people will simply say, " It can't be the truth because they haven't tried to hide it". "It's there for us all to see".
My own view is that this has been done many times before now and the public haven't believed what I believe has been put before them.

I like it, hide it by not hiding it. It's perfectly paradoxial and yet makes perfect sense. Use the fact that people are certian that you are hiding the truth and show the real one while orcistrating a fake one under the lid.