It ought to be obvious: Do things that require gold before you dump excess gold into guild donations and astrals. However, we goof up so often on this that we thought we'd make a checklist.
You need gold to do these things:
Once all those things are done as best you can do them right now, then dump your excess gold by doing these (in order of priority):
About Gems: (Almost) always SYNTHESIZE before CONVERT. Let your gems grow as they will until they're at the level they need to be for you to make a gem that you can socket. The one exception might be if you can convert a low-level attack gem and set off a chain reaction that results in a socketable gem many levels higher; then you'll get that BR boost a lot sooner.
About the Guild Wheel: Doing a full run on the guild blessing wheel is our number one priority for each day. It's the biggest source of zero-experience powerups that we know of.
Our Number 2 priority is getting to level 80 devotion where you get that extra level 2 gem. A level 2 gem doesn't sound like much, but it adds up to 2 extra level 4 gems each month with zero experience cost.
Guild Skills vs. Astrals At all levels, increasing Guild Skills will give you more Battle Rating per Gold than astrals, so we just get some decent astrals (purple and gold, level 3 or 4) and then focus entirely on powering up guild skills before we try to boost our astrals way up. As of today, March 25, 2014, all three of the S1 Megabrats have all level 10 Guild Skills.