PHB p208
"Instead of suffering penalites to abilities, the subject takes 3d6 points of damage each day he does not attempt to follow the geas/quest."
Geas is on a per day basis, so its not worth much other than long term goals "Recover the Sword of So-And-So and bring it back to me."
If the person you geas doesn't like you, you'd best have a teleport as well to get out of Dodge before they kill you and bring your corpse back the sword.
Geas is easy to get around nowadays, but since theres no save it had to be that way. Its utterly worthless in combat in most cases, since its easy to rationalize ways out of it, or just take the hit once.
"Cause me no harm." -they kill you take the damage, and leave... odds are they wont harm you tomorrow since you're already dead.
"Obey my every command."-see above
A fairly safe one might be "Keep me alive and do nothing to hurt me." Since they'd have to keep you alive for the duration, though odds are they'll just kill you once the spell expires (and you're bound and gagged until then).