For those unfamiliar, a staple skill is a skill that most builds will incorporate. This excludes very thematic builds that must use a bunch of different things to achieve their goal.
The initial, main 3 that jump out at me first are:
1) Mantra of Conviction - Overawe
2) Breath of Heaven - Blazing Wrath
3) Deadly Reach - Foresight
These are easy all-around damage buffs, so most builds will use at least 2 if not all 3. Obviously, if 2 Monks are in your game, one would use MoC and the other would use one of the other Mantras.
As far as adding more, it gets tricky. Some may consider Sweeping Wind a staple, splashable skill -- it's easy to have the spirit before a fight, and in harder difficulties or versus bosses, the damage is constant and free.
In PvP, other skills come into play. In PvP, I would consider Serenity a staple; some would consider Seven Sided Strike, perhaps runed for Sudden Assault, a staple.
People have been posting builds for a long time, and excluding things like "WoL spam" or "tank monk," almost every build uses those top 3 skills -- and with good reason. You could even create a skeleton PvE build like this: