"Not get hit" is indeed the best way, but obviously it's not a very productive advice.
Visiting the nearby town and securing a supply of peasants is the most direct approach. They are... well, hopelessly bad at first, but it's better than nothing, and they're very cheap to replace. You can make them somewhat useful if you give them shotguns or small throwables (there are other ways too, but I'm keeping it simple).
You start with a parrot. It's easy to forget, but it's there and can fill one slot. Not a combat unit by any means, and prone to die, but again, it's better than nothing and it can scout good.
Look out for missions like "Save a Sister", where you can find extra Gals. They mostly aren't easy, though.
None of these methods are perfect and often you will face a dilemma: send an incomplete team or just skip this mission. Deciding which to choose is a matter of some experience and learning which missions are easy enough to do with a smaller crew.