It's not easy to find good tenants, however, I do believe that a few factors will influence how the tenants will treat your property.
If at all possible, I try to choose tenants who do not have children and who do not smoke. I'm not anti-children, I have one myself, but they can and do cause significant damage to furnishings and fittings. I also wouldn't use an agency as I'd rather meet the prospective tenants myself - my husband usually gets involved in this part of the process as he's an uncanny judge of character.
It's also important to tell the prospective tenants upfront exactly what you expect from them in terms of cleanliness, rent payments etc. If you're fair with them, more often than not, they will be fair with you. We have a list of issues we go through with them before they sign the lease. The first lease is always for 6 months, just in case we're not happy with them.
When a property is to be re-let, we make sure that it is absolutely spotless and any damaged/soiled furnishings are replaced. If the tenants take on a property in good condition, the hope is that they will maintain it that way. The furnishings are also important in that they should be easy to maintain, eg. wooden/tiled floors, leather suite, etc.
I treat my tenants very well - if there's a problem, I make sure it's sorted to their satisfaction as quickly as possible and do my best to keep them happy.
Our best tenants at the moment are two Polish couples sharing a 3 bed house. Rent is paid bang on time, the house is spotless and the lawn looks better than mine! We're very lucky and value them accordingly.