They are hard to do as it is often difficult to know how players will react, if not, impossible. Try building a team of a certain personality/trait (eg. Determined or Professional) then you figure out what works for them. It never will be 100% but I haven't had any negative reactions in my current save. Having said that, I have only had 3 team meetings in 5 seasons. They normally take place in the last couple of weeks to the title run in to make sure we don't drop our heads. It's more motivation to avoid complacency than anything else.
To try and get your players to turn around a losing streak, then you have to go against what you really feel. Most of the time you will be annoyed and just want to yell at them, which will in turn be even more negative. Players know when they are underachieving so yelling at them won't do much good, try and encourage them.
I would love more improvements in player interaction in the future as at the moment, it's kind of hit or miss.
clinging to a rubber duck in the middle of a raging sea....
I would like option available at all times, not when a programmer has determined it should happen. Many a time I have wanted it to boost morale before a vital game. Would also like addditonal options such as "we are about to change tactics so things may be tough for a few games" or "we are three wins away from promotion/playoffs/european spot lets just keep our composure".
These are only worthwhile when morale is rock bottom and there's nothing to lose. Encourage the team, ask captain, ask room, end meeting. Then a no pressure team talk for the next match and it can often get you out of a slump.