Almost every dispute over a security deposit is a dispute about what the flat looked like when the tenant moved in. The tenant remembers a stained wall, the landlord remembers a freshly painted one, and neither has evidence.
Whoever has dated photographs generally prevails. So spend an hour on the first day, before the furniture arrives:
It is an awkward conversation for ten minutes and it settles the last day of the tenancy. Also check the paperwork before you take the keys: a signed rent agreement with every page initialled, and a receipt for the deposit.
The free tenant move-in checklist from My Property Pages has 6 sections and 45 checks, and you can print it or edit it in Word.