For a series to work well, it is important to recognize the role of the Spirit and have flexibility in scheduling. Here are two more important issues to consider:
Avoiding the sameness – A long series in the same book can get old. There are several ways to avoid this. Vary the message structure (include a first-person sermon, a more narrative sermon, a good old clear deductive sermon, etc.) Vary the text length (some weeks you may need to cover only a few verses, but other weeks it would be possible to cover a chapter or two). Perhaps sameness can be avoided by having another speaker involved (make sure all speakers in a series are on the same page regarding the books overall idea, structure, etc.). And, of course, a long series in the same book can get old, so . . .
Length of series – Think through the length of the series. The old days of seven years verse-by-verse through one book are the old days. Today some advocate that a series should not go longer than 8 weeks. Others say 4 or 5. I say you have to think through the situation – who is preaching, to whom, what are they used to, what is the preacher capable of doing effectively, what is the subject matter, etc. No hard and fast rules, but several months will probably get old for some. Cover ground more quickly, or break the series and then return to it.
It is a good idea to usually preach sermons in series. It is a good idea to think through your series seriously.