Use lemon juice to write sigils everywhere. Doorposts, doors, walls, cabinets, books, book pages, work desks…
The “ink” is invisible, so the possibilities are endless. What’s more, lemon juice is generally safe on most surfaces (aside from brass-plated items, which it damages) and smells nice too.
Bonus points: lemons are associated with cleansing and purification; which can add an extra little boost to your sigils, wards, etc.
