It is hard to be honest and tell someone there point of view or idea is really bad without hurting there feeling. One way you can do it in a constructive way would be; "I thought your idea was ok, but I think we could come up with something better.' This tells the person that there idea was not very good, but still gives them the confidence to try to do better. I hope this information helps.
keep the line on the issue and not the PERSON. And listen wholeheartedly to that person. Do not be quick to speak cos the other person knows it when youcannot wait for him or her to finish so you can talk back