Listen to what they want - my oldest wanted to talk over every exam in minute detail, the younger didn't really even want to say good/okay/bad
Let them make their own plans - rather than pushing them to what you think is best, unless they ask for your help in which case you can offer suggestions.
Don't tell them that effort is all that matters - if they're a hard worker, they care about results, you pretending they don't matter fools no one
Don't keep asking about how much work/what they got in the last test if they are being cagey - that can make them feel under pressure. If they're keen to tell you, then great, but if they're not, leave it.
Do produce little treats for while they're revising - packet of Jaffa cakes makes revision seem much easier
A break is good, but on their terms - you can suggest things, but an hour of trailing a sulky teen outside because you have deemed it good for them won't actually help their revision.
Encourage them to have a day, or an afternoon if that seems too much, off even through the exams. Having a day where they don't have to think about what work they should do makes them much fresher. We always had the rule of no work Sunday at home. Occasionally there was a reason why they needed to work Sunday, in which case they'd take Saturday off. As adults they say they appreciated feeling they had a day where there was no feeling that they ought to be working.
But mostly, give them space, be led by them, and let them know you love them and will be there for them whatever.