He will be at great risk just as they are and, since it is their duty to support him, they will earn their glory (dead or alive) by doing so; he will look after his own soul and they should look after theirs. This establishes a feeling of equality of purpose since, even if they do not understand why Henry is fighting, they only have to recognize that he is doing what a king must do (and will worry about the reasons for the war) and they are doing what subjects must do (and will worry about how best to do their parts).
In terms of planning Henry had to make the best of two bad things. First, his force was smaller than the French army and, second, he was fighting away from home, deep in French territory. From the beginning he decided to minimize damage to his army. His argument with the Governor of Harfleur shows how he logically convinces the city to surrender in order to avoid being plundered. This saves men for Henry and ensures that the French whom he defeats along the way do not bear too much ill will toward him. His organization of the effort relies, in large part, on his policy in forbidding anyone to steal and requiring that everything be paid for and that the French people not be abused in any way by his men. This makes sense since it is his intention to rule