Shielding effectiveness = reflection + absorption
Shielding effectiveness (in dB) is made up of three components: reflection loss (R) at the interfaces, absorption loss (A) within the material, and a correction term for multiple reflections (B). Rule of thumb: at low frequencies reflection dominates, at high frequencies absorption does.
Electric vs. magnetic fields
Electric fields are comparatively easy to shield. Low-frequency magnetic fields are the hardest to shield: small absorption loss and low wave impedance lead to low reflection loss.
Choice of material
Well-conducting materials such as copper and aluminium deliver high reflection losses for electric fields. For low-frequency magnetic fields, by contrast, ferromagnetic materials (e.g. steel, mu-metal) with high permeability are more effective.
Thickness helps — but selectively
Absorption loss increases with material thickness (in skin depths), whereas reflection loss is largely independent of thickness and depends on the impedance mismatch. Often more important than thickness are slots, gaps and feed-throughs: an opening acts like an antenna and can render even the best shielding ineffective.