Anything that is coming from a wet part of the plant should be dried before storing. That doesn't guarantee that they won't go bad, but it helps. I haven't tried cucumber yet, but my tomato seeds have been okay.

Also, I keep them in the fridge, which basically simulates winter. So when the seed is suddenly warmed up and wet, it is likely to sprout.

Some seeds, like strawberries or roses, are weird and need a special process to sprout.