ive also found simply not participating in certain topics as well as saying "is that so?" or "maybe"..."maybe your right" are great to use
I'll add in my two favorites:
"okay" (said in a neutral or accepting tone of voice - can't argue with someone who won't provide any resistance); and
"that's certainly something to think about" (again, can't argue when the other person provides no resistance)
The latter of these two works particularly well in a professional setting, as you're not denying the validity of their comment, but neither are you committing to take any action on it.