Keep your willpower above 60, obviously.
You gain -1 willpower from each point of corruption you have, so try to have as little built-in corruption as possible to start with.
Secondly, try not to gain any of the curses you can avoid. Eyes of Greed is easy to manage though, just don't buy anything at start of day, do your shopping at end of day.
Thirdly, and this is kinda the big one, there are a few lewd event chains that can lock you to low willpower. If you do both Hermann and Roland events, gaining those stacking willpower reducing passives, you won't be back above 60 willpower without consumables. If you want to play a lewd playthrough, but still clear the game, you can't really do all the lewd options.