How to keep fresh
You can't, really. Not for more than a few hours...The edges will go stale and if you use anything with water, like cucumber, it will make the bread soggy. For a few hours, I would just put them in a tight-sealed container or wrap them with plastic wrap to minimize the air exposure.
How long are you trying to preserve them? Maybe you can keep the components separate and make them right before serving?
Keep the prepared sandwiches covered with barely damp paper towels. That helps keep the bread from drying out. I agree with lloreen, prepare a few hours ahead, but not much more.