Mass-effect – change the Galactic Readiness in the game by editing the configuration

From the feedback on my website, it appears it does have to do with the Platinum afterall.

If you have 250,000+ total resources with at least 15% being platinum, you get the full 100 point bonus. You can also get 10, 25 points if you have less than the amount required for the maximum bonus of 100. I cannot clarify this but a few people have mentioned it elsewhere.

Source: http://www.gamersheroes.com/mass-effect-3-war-asset-guide

All of this is based on observations, primarily by users here who have shared their first hand experience.

Galactic Readiness Decay seems to have multiple rates, depending on your readiness score, or perhaps on the source of the score.

In general, it appears to decay at a rate of 0.15% per hour or around 1% per 8 hours, or 3% per day to a minimum of 50%.

There have been some reports of up to 6% per day, though I haven't seen this number echoed elsewhere.

If your GR is low, between say 50-60% then it appears to slow the decay rate to around 1% per day.

It takes very little to beef this back up, if it is causing a problem for you. For example you can play multiplayer, by yourself, and complete the Geth mission ... it's fairly easy to achieve a bronze or silver victory.

As @Edwin notes, if you simply start and exit a game, it will not count as an active day. You have to finish the game, which is why I noted Bronze or Silver being fairly achievable on some missions.