Don't cut them too short, please.
Stress 'ta-GLI' — subjunctive imperative form. 'Troppo' = TROP-po.
Say this clearly before the hairdresser starts cutting, especially if you're nervous about losing too much length. Being specific prevents misunderstandings.
'Non li tagli' uses the formal imperative (Lei form) — polite and appropriate in a salon. 'Li' refers to 'i capelli' (them). 'Troppo corti' = too short (masculine plural agreeing with 'capelli'). If speaking informally you'd say 'Non li tagliare troppo corti'.
Solo le punte, per favore.
Just the ends, please.
Minimal cut request.
Non più di un centimetro.
No more than one centimetre.
Precise length instruction.
Voglio mantenerli lunghi.
I want to keep them long.
Stating your length preference.
Italian hairdressers are known for their precision but also for their opinions. It's common for an Italian stylist to gently suggest a different length or style ('Le starebbero meglio un po' più corti' — they'd suit you better a little shorter). Italians generally welcome this professional input rather than finding it intrusive.