Make sure every piece of gear they use for cleaning is the best you can find, repaired and always working at it's optimum. Pay them a bit more than they expect (as you are already doing)  and make them know they are important to you and the success of your business. Reward them appropriately. Share gratuities with them that come from your customers and make sure they are aware how delighted your customers are with the job they did for you. 

Treat them with dignity and respect. Listen to their questions and opinions. Train them properly to complete a job fast and easily. Don't forget to convey your expectations.  Anything short of that is unacceptable. Compliment them on a job well done and praise them for anything they did that was above and beyond the call of duty.  Average jobs are just that. Not bad, but definitely not good or great - they are just average.  When they do better than expected, make sure they know you are proud of their work.

As they help you create raving fans (yes, I mean that even in janitorial services) those customers are going to be your new free advertising base that will develop more customers.

It is your serve. The ball is in your court. Make it count or you will fail again.