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They are usually specific to my frame of mind at the time and I don't use any time honoured mantras. Actually they may sound kind of silly but they usually do the job for me.
For example yesterday morning my mantra was "Even the leaves are Buddhas" but it could be anything. "All beings deserve peace" is another one. Anything that doesn't leave questions to be tossed about and that is easy to return to works fine for me.
Threads of the Haiku
The river over flowing
Beavers lie in wait
Tonglen is a major part of my practice, as taught by my teacher several years ago. We do it as a sanga group as well. But you always start and focus on someone close to you who it is easy to be compassionate for, and then expand as you get comfortable to challenging people, then entire groups of people and then eventually all of humanity.
That's the same Metta meditation Ajahn Brahm teaches.
I also like the Tibetan Metta meditation where you imagine taking all the suffering from all beings everywhere. You'd think it would have a bad effect on you but it's just the opposite.
I think that's because we are kind of acting like trees during this type of meditation. Taking in the suffering, transforming it with our breath and breathing out metta.
I usually come away from it fairly refreshed.
There is no truly solid ground.