Question: What are all the ways to increase the readiness factor for the war with the Reapers?

Answer: 
As far as I can tell so far, the only ways to increase galactic readiness is through multi-player or either of the iOS Mass Effect 3 games.

You should be able to acquire enough military strength in game to where the readiness doesn't matter.
The only two ways of increasing readiness is by participating in Multiplayer, and by trading in Intel from MEI (Mass Effect: Infiltrator).

MED (Mass Effect: Datapad) is a FREE app which allows you to view your Galaxy at War.
Which in turn is split into 2 things, Strength and Readiness.

I have explained readiness.

Strength is increased by you war assets in single-player, which in turn is multiplied by Readiness.

The better Strength/Readiness you have the better the ending is (Although i don't know what it is I've heard it sucks). Still if it sucks, why get one which sucks more :)

Unfortunately i don't know the specifics of percentages or whatever as i do not have the game yet, however i hope to be the proud owner of a copy soon :D