Hard to say without more info as to what plants, where they're growing, etc., but usually, brown tips is to do with dryness, or heat, or cold.

A combination of saline water and poor drainage can cause brown tips, but, as Bamboo says, drought (wilting between soakings) very dry air, and wind are more common causes. Cold or heat are also possibilities, especially if they haven't been hardened off before being brought outside.