Beware of whirlpool when sailing, which are invaluable for traveling but will DESTROY half of your top stack creatures. An ideal strategy is to split your weakest troop into stacks of 1, and let those single-troop stacks perish to the whirlpool any time you need to use it.
Discussion and techniques
Some things to keep in mind with your troop stacks are protecting your ranged attackers, first strike opportunities, and sometimes - allowing a support hero to survive via troop splitting and retreat.
Ranged attacker stacks are typically best at the top or bottom of your troop alignment because the can take advantage of the corners of the battlefield and take less effort to defend.
First strike opportunity strategy involves keeping your very quickest units (typically flying creatures such as dragon flies or angels) in a central slot in your troop alignment as they'll have a slightly increased chance of reaching the enemy in the first round.
Troop splitting defense is a technique involving taking at least 7 low-level creatures (such as zombies or iron golems) and splitting them into very small stacks, minimizing the chance even a powerful opposing hero can defeat your support hero in one round. This will allow you to protect your artifacts, re-hire your hero (for 2,500 gold of course) and even get a cheap shot ranged attack or spell in before retreating.