I'm not a psychologist, but in my line of work (police dispatcher), I've had to learn how to speak to victims of crimes and other folks who are facing crises of one kind or another. I have found that, for the most part, if you can put yourself in someone else's place and think about what kinds of things YOU would want someone to say to you, it will go a good ways towards helping you to frame those words.
Of course, doing Tarot readings and taking phone calls at a police department are two different things. For starters, your goals are somewhat different; call-taking you're usually trying to get information so you can send the assistance needed. Whereas when doing Tarot readings you kind of are the assistance. You may only be the first step for some folks, but you're still trying to help.
For the most part, you don't want to come across as antagonistic or accusatory (yes, I have heard of Tarot readers doing that). Remember you're trying to guide them towards the answers they seek, so use phrasing that guides rather than denigrates or accuses. On the other hand, sometimes a person might need a firmer hand ... kind of a wake-up call. I have found that most of the time, the Tarot reading will guide me to what kind of tone I need to use.
I guess what I'm saying is that common sense will help you quite a bit. If someone is actively suicidal, you're not going to tell them "go ahead and jump then." However, if they are suicidal, you don't beat around the bush too much either ... getting it out there and getting them talking is a good first step. This applies to many situations of people in crisis. Avoiding the subject isn't usually what they need. But follow what the cards are telling you to say.