I've just tended to play around with it on the demo, the option that works for me the most is that the pre match team talk I usually talk in a calm voice and tell them its a winnable game. Second half if we're winning I talk in a passionate tone telling them that i'm pleased and to keep going.
If i see any players reacting badly to my team talk then i speak to them individually and tell them i have faith in their ability in an assertive tone.
Also the thing I think helps get a good reaction is to look at what your coaches think you should say to them, go through each of them and look at their advice and then pick the one that seems the best. If you losing and your coaches tell you that they're playing poor, then tell the players in an aggressive tone that they're playing poor.
It's hard to judge, but as i said i'm just playing around with it and it might not work for all teams but i've had some good solid talks.
If you ever go on a bad run or anything then organise a team meeting and talk to them about it and then check if anyone wants to add anything and then usually a player tells you if there's a problem and then you just solve it there and then xD