Your maximum willpower is based on your stats, plus bonus points for certain events. It's calculated by: smarts + stamina - (sex obsession + submission) + bonus, with a floor of 1.
Add to your normal max willpower by getting bonus points:
During hypnosis sessions, if you choose an option that requires exerting willpower (can only be done once per fetish)
Choose the voluntary route for unisex clothing
Choose the voluntary route for female clothing
Ask the manager to stop during the phase 1 "ending" event
Earn willpower points:
When you try to recover data without assistance
If you push through and deliver lunch even after food has been spilled on you
By spitting out the candy
If you exhaust all discussion topics with your psychiatrist before leaving the session (repeatable each session)
When you meet the lingerie shopkeeper the first time
The first time Bob sees you in female or unisex clothing (if you get the female one first you can't re-trigger for unisex, but you can get unisex, then female)
By putting yourself into chastity voluntarily
By figuring out the uniform portion of your contract
During hypnosis if you choose an option that requires exerting effort (must be at <50% fatigue)
Being over 50% horny will result in an erection unless you're in a chastity device. Even then, being too horny will result in the character slowly leaking precum, which can stain your underwear.