@Kenneth Garrett
I agree, there's a fine line you walk with being a push over and letting tenants take advantage, and having some compassion in the name of keeping a good tenant. Someone who keeps the place clean, and has a good history of payments, is someone that you want to try and keep around if you can.
If the tenant had a short rental history with you and/or has some other issues, then yes you should take a hard stance and explain the agreed upon payment terms. If they fail to live up to those terms, then you either try to get them to move out on their own, or you begin the eviction process. In most cases, you really do have to be firm about payment terms, or tenants will take advantage. However, in some cases its worth putting some effort toward keeping good tenants in place.
I setup my leases so that they go month to month (with 30 days notice) after the initial 1 or 2 year lease period. That gives them the flexibility to move if needed, without signing a longer term lease, but more importantly, gives me the latitude to end the rental agreement if there are issues like payments, maintenance, etc.