Just remember that these are strangers who are not your friends and that you are in a business arrangement. Guests will often try to take advantage of hosts, asking for perks such as discounts, letting them check in a bit early or check-out a bit late. In more extreme cases, I have had guests do things like ask if I could watch their bags for a day or two after checkout, saying that their flights are delayed, or asking can they keep food in my house until they come back next time, or drive them to the bus/train station/etc.
The best advise is to simply have very firm rules and stick to them and, if possible, have them be automatic. In my residence, we have an electronic door code that expires at checkout time. If a guest wants to stay late, they *must* pay a late checkout fee or the door will simply remain turned off and if they are found in the house after that they are asked to immediately leave. Since our door code is on a timer, this removes most talk about doing guests a favor, let us back in for a minute, do us a solid we forgot something, etc etc.
The same thing about leaving things in our house after checkout. Guests are advised we have a housekeeper, rooms are thoroughly cleaned, abandoned food thrown away and bags considered abandoned if a guest leaves them in the house after checkout (we do make exceptions for things accidentally left behind which we are contacted about, but do not hold bags or luggage for guests).
Just remember, you are not their buddy and not their pal. You are their hosts, you are being paid for a service, and just like any business there are procedures and rules that must be followed.