Biggest thing I found useful was to listen to your casters. People want to see what the casters are talking about.
If there is no clear hint of who to follow from the casters, think "who is likely to go huge very soon?". E.g. one team got forced, other team has full uber, first team's roamer is likely to do something to force the other team.
Keep an eye on the classes to make sure you catch offclasses.
Don't switch POVs too quickly. I'd make an exception for this if you accidentally switch to someone respawning which isn't that big of a mistake anyway. For example, if there's a x2 backcap, there are 2 or 3 likely classes it could be, chances are you could switch to a scout grabbing health way behind the action whilst the other is capping but whatever shit happens.
Sometimes switching to the medic's POV is better than trying to guess what's going to kill the medic.
Above all, concentrate on the game. Get into it as much as the players are getting into it. When I streamed, I had information 20 seconds ahead of what was happening, so if someone bombed the medic and got the uber drop, I'd know 20 seconds in advance and I'd be able to switch to the person. However, in some games I'd find myself not using it at all but still catching the action because I was so focused on the game.
Finally, remember that you can't catch every single bit of action. One particular moment I've experienced this was during this game (i think) when both roamers got huge plays at pretty much identical times. You're always going to miss something, don't let it get to you and carry on following the game.