Consider using this right after an item spell, as the sudden loss of energy from losing your weapon will give you a huge energy deficit.
Keep a low-energy set on hand. It can be used to hide energy to help meet Ether Signet's requirement.
Ether Signet checks that you are below the energy threshold after it finishes activating and after any other effects occur. Energy regeneration, Lyric of Zeal, Scribe's Insight, etc. could raise you above the threshold, resulting in no energy gain.
Heroes give high priority to this skill as soon as their energy qualifies, but they fail to take energy regeneration into account and thus usually waste the signet. However, this makes it a great skill for Hero Bonders.
The following table shows the maximum energy gain you get from this skill over time: