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

Answer: 
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
In multiplayer, play bronze and win to get about 4% added on to your readiness. You get much less if you die.

As for singleplayer, talk to people and do side missions.