For the inviting church:
1. Give him guidance about what to preach on. Or not to preach on--it's awkward to be told 'Preach on whatever you want' only to show up and discover you're preaching the same text as the previous week.
2. If he wasn't your first choice to pinch hit that day, don't tell him.
3. Tell him what time to show up, and how long he should preach for.
4. Tell him who is going to greet him, and where. Be sure he has directions to the church. Tell him about any road-work to avoid.
5. Let him know what kind of mic he'll be using, and if he has a choice, ask his preference. Explain how the mic works when it is given to him. Tell him if powerpoint is an option. Tell him if there will be a podium, pulpit, or nothing. Give him guidance about dress code. In short, don't assume anything; err on the side of giving him too much information rather than too little.
6. Don't ask him to administer a sacrament. Do the sacrament another day, or ask an elder or pastor of the church to do it. He is in a strange place and doesn't know the particulars of your church's practices, which for you seem totally normal and obvious. Let him focus on the preaching event.
7. Send him a PDF of the bulletin or some kind of write-up that outlines clearly the flow of the service. Tell him exactly at what point he will be getting up to preach, where he will be sitting for the service (front row? chair on stage? anywhere he wants?), and what the cue will be for him to get up to preach. Let him know how he should end (pray? invite someone else up? a brief interpretive dance?), and what will happen right afterward. If you expect him to give a benediction tell him.
8. Pray with him before the service.
9. If the passage on which he is preaching is going to be read by someone else in the service, tell him beforehand. Sometimes the preacher builds the Scripture reading into the opening of his message in such a way that creates awkwardness of transition if he has to excise the Scripture reading from his sermon at the last minute.
10. Identify for him what version of the Bible your church normally uses in public worship, and how important or unimportant it is that the guest preacher stick to that.
11. After he preaches, even if it was the worst sermon you've heard in a long time, find some way to affirm him. And do so by identifying something specific in his sermon that helped you. 'Good job!' is pretty much a zero response. 'I was helped when you said...' is what he needs to hear. If he invites it, offer critique. And through it all remember that it's much easier to criticize preaching than to preach.
12. Tell him beforehand how much you're going to pay him. That is not unspiritual. It is blessed clarity. He deserves it.
13. Pay him well, a la 1 Cor. 9:11. Honor him and his family, if he has one, in that way. Obviously there are no hard and fast rules to lay down as churches and social contexts differ so widely. But I think it would be a good rule of thumb, in a church of 100 members or more, finances permitting, to pay the man 1% of the approximate average annual salary of your members. I.e. if the average household income is $50,000, that would be a check to the guest preacher of $500. Another way to look at that is that you're paying him for about half a week's work (52 weeks per year is 104 half-weeks per year--and $50,000 divided by 104 is about $500, too). If there is more than one service, adjust that number as you see fit. If he has driven a distance to join you, cover that expense.
14. Follow up the week after. Thank him. Even if you intend never to invite him back, thank him and encourage him, as you are able to do so with honesty.
For the guest preacher:
1. You are not there to impress them. You are there to help them.
2. You are not there to outshine their regular preaching pastor. You are there to help them.
3. You are not there to make a little extra cash. You are there to help them.
4. Time permitting, do something fresh. Don't pull one out of the Sermons Folder unless you have to (or if they explicitly encourage it in the invitation, knowing you are currently pressed for time). It is harder to prepare a fresh message. But much more fun, more powerful, more meaningful, less robotic for you, and generally, I think, more edifying to the listeners.
5. Every church has its own theology, its own ethos, and its own view of preaching. Nevertheless, preach the gospel. Don't give a pep talk. Don't give advice, like I am here. Just tell them why Jesus is better than they think. Prepare your message with Luke 24:27 and John 5:39 and Acts 20:24 written out on sticky tabs stuck to your laptop. Whatever you do, give them gospel oxygen. Let them breathe again.
6. You are not there to change their church in any kind of structural or theological way. Even if they need it. Nor to make subtle suggestions. That's not the role of a guest preacher. You are there to encourage and edify them on whatever points you and they agree on. If you can't find enough central things to agree on, decline the invitation.
7. Don't say yes out of obligation (it's hard to preach grace when you do it out of a law-mindset) or because you think you might never get another chance to preach (if God wants you to preach he'll open up the doors) or because if you don't preach then they're missing out (they're not). Say yes if you have time to prepare well, and out of a glad sense of privilege, more confident in the power of the Word than the power of your persona.
8. Whether you agree to preach for them or not, thank them for the kind invitation. You don't deserve it.
9. Don't be scolding. There is a time for a certain kind of scolding, perhaps, when immaturity must be lovingly confronted (1 Cor 4 comes to mind). But not from a guest preacher.
10. As Calvin Miller recently wrote, it is just as important to know your context as it is to know your text. Seek to know the history, especially recent history, of that church.
11. As you leave, remember two things: (1) you are not as great a preacher as you think you are, and (2) your preaching is more effective than you think it is.