depends on the advice , otherwise you should be humble and honest without threat
As a manager, I would just give my manager advice straight out. That is what I am there for, to help my manager do their job better by providing information they may not be aware of or have not considered. Beyond that, it is their call.
Ask their opinions on what they think the outcome would be if an issue or requirement was handled differently.
I usually ask for a private meeting, open, sincere and with a early preparation of the themes. First I ask to my boss if this situation of advises context in a talk is possible.
Make him/ her think they thought of it
Pull them to aside from other colleagues
Are they asking for it? are you that good of friends out of the work place? If no to both, I would keep it to myself.
Present your idea as a solution to a challenge - not as a boss problem
Ask if you can give him a suggestion
Anonymously ask a question and let the Comparably Community respond with real answers.
Ask an Anonymous Question