The only way to really keep the respect of your students is to have a mixture of both praise and criticism. The most successful teachers are not the ones who are negative all the time. The reverse is also true. The overly positive sugar blowers eventually lose respect as well.
Excessively strict and critical teachers will never reach their full potential. They may be able to intimidate students into behaving, but that isn't exactly the best learning environment. So, the answer is to never criticize and be negative, right? Booooo. Wrong.
When teachers(or parents, or bosses, or any other kind of supervisors) blend praise and criticism, they show that they are willing to be honest. This makes both kinds of comments more believable. If all you do is tell people how great they are, eventually they will catch on that you are full of it. Most people respect and appreciate honesty.
This is true for coaches as well. The coach who does nothing but criticize will lose his team quickly. Smart coaches look for ways to compliment players to avoid doing nothing but pointing out mistakes all the time.
Be intentional about pointing out the good as well as the bad things and you will be a much more believable and respected leader.