For me there are 3 factors in communicating with players:
1) What you communicate (what message am I wanting to get across? What value does that message add to how I'm running the game?)
2) How you communicate (body language, tone of voice, word in passing or pulling a player out to talk to them)
3) When you communicate (at what point do I feel it's appropriate)
Personally I think that often point 3 gets forgotten about. One of the things I see with new refs is that they only tend to communicate when they are challenged by a player and therefore they are doing it almost from a defensive position.
I found an easy win is to be vocal at throw ins. Simple 'just by the floodlight for the throw please number 3' followed by a 'great thanks' when they've delivered it from where you've asked helps your match control incredibly.
In this instance:
What - you are telling the player where you want the throw in taken from
How - Its done in a really polite, non confrontational manner and you tag on an acknowledgement that they've done what you asked with the 'thanks' afterwards
When - at the throw in, 99% of throw ins are not contentious, therefore you're subtly asserting your authority on the game without being aggressive.
Some other ones I may or may not use:
Lots of pushing and pulling at a corner - delivered before the corner is taken - 'Lads (I mostly do men's football, insert other genders in here if applicable), it's Saturday afternoon football, not WWE, let's give it a rest and keep it clean in there please'
Two players having a bit of a scuffle (no cards needed) - 'There's not much in that, but you've both made a bit more out of it than you need to. You're on my radar now, so I'd suggest you sort it out before I really need to get involved'
Just for
@Max2 (as I know it's his favourite) - When sending off a totally obnoxious manager from the technical area - 'Cheerio'
As always, context and game temperature is important, trying to be too clever in comments to players or managers can easily backfire on you. I've got one manager on the league I'm on that I've tried all approaches with to get him to behave, the only one I've found to work is go and give him a card the moment he starts and follow it up with another if he carries on - banter, a firm word, reasoning with him, none of those approaches have worked for me with him, however I know other referees who can manage him in a different way.