A worship leader's biggest challenge is always choosing songs. Regardless of whether it is for a Sunday morning service or just a small group Bible Study the challenge remains the same: picking songs that are appropriate for the setting. Here are a few simple guidelines I follow in selecting songs when I am leading worship:
Does the song's lyrics reflect sound doctrine? Many worship songs are very shallow and don't convey sound biblical teaching. There are plenty of songs that are very popular but not very deep theologically speaking. Over the years I have discarded a number of songs that I had sung previously because on closer examination I have discovered that they were not teaching sound doctrine. Each song should be examined carefully to determine whether what is being said is true when measured against the standard of Scripture.
Is the song one that me (or my worship team) can play and sing? This is an area that can trip up many a worship leader. It's important to keep in mind the skill level of your team (or yourself if you're leading by yourself). Often a worship leader will hear a song and think that it would be a great song for the congregation to sing. But the skill of the team might not be such that they can lead the song.
Or consider someone like myself who leads worship in a small group setting. My worship team consists of one person: me. This limits to some extent what songs we can sing. It has to be a song that can be done with me just playing my guitar.
Another consideration is whether the song can be sung by the team (or in my case, me). In order to effectively lead a song I've got to be able to sing it so others can follow. And I have to be able to sing it in a key that everyone will be comfortable with.
Don't worry about themes or trying to fit the songs to the message. There are different schools of thought on this. Some try to tie the songs to the sermon and others don't. I've typically been in the latter camp. If I can tie the songs thematically to the sermon then that's an added bonus. However, it's not something I normally strive for. But there is nothing wrong with trying to do it if you can. It all depends on the sermon and the topics that are being dealt with.
On the other hand, some songs are grouped naturally together to fit a particular theme. Sometimes the worship sets I put together are tied together by a common theme. But as I said before, this is usually not my intention. I don't try too hard to make everything fit a particular theme or message.
Don't plan on more songs than you have time for. This seems like common sense. But it is an important point to remember. It's crucial to cooperate with your fellow leaders to work out a mutually agreeable amount of time for worship music.
Plan ahead. I find it helpful to start working on the next week's worship set as soon as possible. For example, if I have led worship on Sunday I am already starting to think about the next Sunday's worship set on Monday. Part of the reason for this is you never know how long it will take for your worship set to come together. Part of the reason is also that it gives you the freedom to make changes as the week progresses.
Be flexible. As stated above, things can change during the week. Something can happen to throw off your entire plan for a given service. You should not be so married to your worship set that you don't have the freedom to make changes (sometimes even right before the service is to start).
Don't think too hard about it. I have found that sometimes the more I think about a worship set the harder it is to pull together. Certainly you want to put some thought into it but not so much that you are not open to what God has in store for you. Some of the "best" worship sets I have put together have come to me at the most unlikely times and when I have been thinking about it the least.
Pray. Finally, and most importantly, you should be praying over every aspect of your worship. Pray that in leading you will have an attitude of worship in front of your congregation. Pray for your team members. Pray for your pastor. And pray for your congregation.