"...Speak ill of others and it will surely come back to you. The mountain does not speak ill of the river because it is lowly, nor does the river laugh at the moutain because it cannot move around"
I read this in Tohei Sensei's "Aikido: Coordination of mind and body for self-defence", it's an unattributed quotation, so maybe it is a proverb, or maybe it was said by Tohei Sensei, or O Sensei, I don't know... but I like to apply it when I can. Of course, often I will have negative opinions of people or things, and sometimes I voice those opinions; I'm only human. I do try to voice them as respectfully as I can, however. Particularly valuable as I guide for dealing with conflicts I find "Aikido in Everyday Life" (Dobson/Miller), which talks about how to deal with arguments and other such things as if they were attacks in Aikido; be aware of them, avoid them before they happen if such is possible, if not harmonize with them, let them pass you by; not in the bury-your-head-in-the-sand way, but in a way that resolves the conflict without damage to either party. I don't explain it very well, and strongly recommend that people read the book if they have not already.
But hey, I'm no saint. But I do try.