I genuinely try (and usually succeed) in forgetting about the score. (I have had a lot of big score lines indoors and out).
Especially as an AR, I find it easy to forget the score and focus on applying the laws. Even if I am noting the score, I shut it out most of the time. So I avoid the temptation to go soft on a losing team.
I think it also helps one you have had some crazy score lines with dozens of goals - in those scenarios teams genuinely appreciate it when you consistently apply the laws IMHO. Of course there are frustrated players on the losing team. I try to keep showing them respect. And I make a point of thanking both teams when one team has carried on trying through a ridiculous drubbing.
No player likes to think you are pitying them. And penalising foul play and infringements, even at 10-something nil, is fair to the players.
The hardest part is when there is kicking the post, kicking the ball away etc. from a team getting thrashed. My tip is to act very quickly. Give a card if it warrants. And make sure a verbal is well heard. I have had a few situations with post kicking and bottle throwing recently where I haven't seen who it was. I made a point of warning the whole bench so both teams could hear.