Many years ago, when I was experimenting with different attitudes, I came across an interesting philosophy; brutal honesty. It is basically that you say exactly what you are thinking, putting aside any politeness in the words.
This should be especially true for negative things. So as an example, if somebody ask about your opinion, and you have a negative idea about it, don't lie or add politeness! (E.g. "That is good", "That is good, but needs work.", "Maybe this idea is better") Simply say what you think (E.g. "That is not good").
This can be very confrontational as it can introduce negative emotions, but I really do enjoy the philosophy of not trying to sugarcoat my words, which in the end dilutes the meaning, and makes you unconsciously feel like a liar, and a pleaser. I believe that it is always better to say what I honestly think, than what I believe the questioner wish to hear! It just seems much more valuable.
I would like to couple this with another concept I also enjoy. Do you remember the phrase "If you don't have anything nice to say, then don't say it."? I would like to paraphrase it to "If you don't have anything constructive to say, then don't say it.".
I put these two ideas together to practice what I call constructive honesty. I like to express my opinion, but I also try to express why I think the way I think. So as an example, when people ask if I like something I would say "No because of Z. Maybe try A!". I'm aware that if I'm fully honest about what I think, then some people probably will not like me, but in the long run this help people (and myself) a great deal. There is no value in a dishonest, sugarcoated, and non-constructive response. Even the people who initially doesn't enjoy my response, later on might reflect more deeply, and find a great use in my honesty (at least subconsciously). The honest constructive response simply helps so many more people than a sugarcoated answer ever will!, by forcing the person to reflect.
Don't try to prevent being confrontational, this stems from a need to be liked. Even worse, it doesn't help the people who genuinely ask what you think. Also, don't be rude, there is no need for that! If you are confrontational, just because you want to be confrontational you are doing this wrong. In that case you are just introducing negative emotions to people that doesn't help them, and you will probably end up with a lot of negative karma.
The advice is that you should be honest about what you think even when it is negative, but remember that it should be constructive! People will appreciate you for it, and you will feel much more valuable in your words, and honest towards yourself.
Disclaimer
Of course you need to be smart doing this. Do not pick fights with people you know will not be able to understand your honesty. In these cases I myself tend to go with a simple "That's cool..." even though I don't mean it. It is simply because I do not wish to fight pointless fights that doesn't help anybody. Luckily, it is very few people who are like this, so in the majority of the cases I try be as honest possible!