There is a very simple reason that I enjoy topical preaching: It makes me look at the scripture from a perspective that I might otherwise miss.
I know there are theologians who insist that expository preaching (using a scriptural text and exploring its original meaning, context, and application) is the only way, or the "right" way to preach. Of course, there are no biblical grounds for that assertion. It's a good way to preach, but not the only way.
I think I prepare for both types basically in the same way. I always start with the scripture to see what it says, what was the context in which it was written, and how was it heard. I understand that what it says, and the context will always be the same, but how it is heard will be different in 2012 than it was when originally spoken or written. I see it as my job to preach it in such a way that the listener will not miss what God is speaking to them through the word today.
If the word was only intended for the original context and for the original hearer, then I could preach from any other book today and leave the Bible on the shelf. The original hearer is dead and gone, and the original context vanished long ago. I just don't believe that the Bible is that irrelevant to 2012.
This week, I had the privilege of preaching from John 4. Our topic was Heroes, and I found that as I read the scripture through the lens of that topic, I saw nuances of truth that would have escaped me otherwise. It was an enriching experience for me, and enlightening for our Transformers.
Do you have a preference of preaching styles?