Materials
Paper, grain and clean folds
The same fold can feel crisp in one direction and springy in the other. On many papers, the difference comes from the direction of the fibres.
Find the easy bend
Curve a spare sheet gently in each direction without creasing it. The direction that offers less resistance is usually the better choice for a long fold.
Score heavier stock
Use a ruler and a blunt scoring tool on scrap paper first. A shallow guide can help card fold cleanly, while too much pressure can cut or mark the surface.
Keep a small sample book
Save one offcut from each material you use. Label it with the project, the paper weight if known, and how it behaved when folded or glued.