Verified answer

I assume you know that with the Free Extended Cut DLC that you are no longer required to raise your readiness above minimum (50%) to get the "best" ending.

But yes, the only ways to raise galactic readiness are through playing Multiplayer and the free app Mass Effect Datapad for Android/iPhone etc. This requires you to link your Origin/EA account with the one you play Mass Effect 3 with, since the app is bugged, you need to repeatedly tap the bottom left hand corner of the screen and when it says you can't connect press back and you'll be in the app. It tells you what to do when it starts up.

Multiplayer. Completing a multiplayer mission increases readiness slightly. The value will decrease after a while.