Clean and dry bore and bolt face. Lightly lubricate (after cleaned) all metal-on-metal moving parts. If your rifle is dirty (interior surfaces layered in carbon) and is causing stoppages or slowing cycling, and there's no time to field strip and clean, you can loosen it up by applying more oil to said parts.
If in a dry arid climate, use only a dimitive amount on the essential moving parts (bolt carrier group) and leave the rest of rifle as dry as possible. Same goes for arctic because the lubricant can gum up and cause problems.
In a cool wet or tropical climate, use generous amounts of lubricant on the mechanism and cover most bare metal parts (exterior and interior) in a light layer. Due to the humidity, condensation and water will collect in the rifle causing fouling and potentially rust. Experience with the weapon system in Vietnam taught this lesson well, the hard way. Frequent cleaning is also required because the build up can be quick and substantial.
That's basically it. Anyone else who knows better feel free to add to this or correct me.