how to make a suggestion

Here are six ways to make a suggestion:

-Have you tried +ing...................?             e.g. Have you tried speaking to your boss?
-Have you considered +ing................?     e.g. Have you considered changing jobs?
-Have you thought about +ing...........?     e.g. Have you thought about reducing your work hours?
-Would it be possible to.............?            e.g. Would it be possible to postpone the meeting?
-Could you............?                                    e.g. Could you ask Jan for help?
-Why don't you...........?                             e.g. Why don't you get someone to check the document? 


If you wish to be particularly polite or you are in a situation where you are the more junior person, it is normally best to  ask if the other person wants suggestions. This is particularly important in cultures that are more hierarchical, as offering unasked for suggestions/advice could be seen as arrogant or even respectless. To do this start the conversation with:
Can I make a suggestion? (polite)
May I make a suggestion? (extremely polite)