There's some kind of formula to figuring out his respect level for you.

Level is determined by money incentive, and attacks previously used. So if he keeps using Daigoro, save and wait until when you pay him, if he uses something better, save, and keep going. If he does not, then soft reset.

Daigoro = minus respect
Kozuka = null, no gain or loss
Wakisashi = plus respect
Zanmato = plus twenty points (I think)
Giving Yojimbo 0 gil = minus twenty points (usually more)

When actually using his attack, the number is added, and plus a random number. If the number is above 200 (or 255, I don't know), he will use Zanmato. It is far from impossible to use, but difficult.

And despite what most people think, having overdrive does not affect his motivation, only how many enemies he can attack. Not having a full bar attacks a random enemy, while having a full bar affects all enemies on screen. But a Grand Summon can't hurt. But neither will it help, unless you are feeling Vegas lucky.