If you just want to become good at pull ups with no other specific goal, bands can help, but do keep in mind that bands will also give you the most help at the bottom, which for most is the most difficult part. Meaning, you might get quite good at pull ups with bands, but as soon as you try without the extra help at the bottom, you get stuck.

Unassisted controlled eccentrics can help if you are strong enough for those. Heavy lat pulldowns can also help build up your back strength.