advice on single renters

One of our flats has become vacant that traditionally has single people renting it. In the past, this has turned into a total mess with people leaving half way through the rental month, people leaving without being replaced or the existing tenants getting unsuitable people in, arguments over who has caused damage etc etc. Can I get some advice so I can start out correctly? eg
leases for each tenant?
remaining tenants responsible for portion of left person's rent?
names on electricity accounts?
who signs the inventory?

thanks in advance