One thing to keep in mind is that player personality does seem to effect how individual players react to the talks. To me, this is the most important reason to build a team of players who all have a similar personality. I find that with a team of mostly professional, determined, or resolute (or anything along those lines), whenever I bring in a guy with a different personality (even balanced, lighthearted, etc), he often reacts completely opposite from the rest of the team when I give a talk. If you have a team with all the same personality, you can get a feel for how they'll react in any given circumstance over the course of a few seasons.
Also, and this is a connection I'm less certain about, but I find that sometimes veterans react negatively ("seemed to switch off") when younger guys react positively, especially when I'm "encouraging." I figure they're holding themselves to higher standards than I am. But frankly, I'd rather the young guys get positive reactions; those old guys are mostly only there for tutoring and leadership, anyway.
Overall, I feel like it's all about managing egos. I'm usually more successful with mid-match and post game talks, because I think about how they played, where they are in their career, who they played against, what kind of pressure is on them, (probably other things too) and then react. I have tangible evidence to go off of. Unfortunately, I seem to do a worse job predicting how they're going to feel playing against a team until we actually get through a half vs. them, so my pre-game talks are conservative, and I'm generally happy to get 2 or 3 players in the green.
EDIT: Also, if you have a team with professional players, you can be disappointed with them even if they put in average performances. I'm not sure what the emotion tabs do, I always speak calmly, but I've told them I'm disappointed on average (6.5-7.0) performances and gotten the entire team "looks fired up" or "looks motivated."
As an overall, don't blow smoke. If they played like ****, let them know, especially if they have good determination. But if they had a solid, gutsy win, definitely make sure to let them know, because they're probably proud of it, and saying nothing on a win where they dig it out forfeits the chance to get an across-the-board morale boost. However, if they beat up on a wimpy opponent, sometimes it's better to say "Good job, but you can do better," or whatever.