I'll share some thoughts since I've been through a recent deconversion...

• First, let go of any notions you have of changing someone's mind. Especially in one or more sittings filled with discussion.
• Second, drill into your mind that typical gauges of the accuracy of a theory/explanation such as tangible evidence, repeatability, and predictability will fall apart in theological discussions. There will always be an apologist to provide a counter-attack for any charge you bring, and this counter-attack will not have it's roots in tangibility, but in a possibility which the theist knows you cannot demonstrate to be conclusively false.
• Lastly, I have begun to postulate that any discussion of reasons a theist might think support their belief will be fruitless, as I'm not convinced many theists realize what actually provides their belief. I say this as 80% or so of believers will have been raised in that belief system. No reasons necessary -- only trust in parents. Only later were reasons a-z provided as a "defense" of the belief. When a theist volunteers some set of a-z as supporting their belief... they may believe those reasons to be crucial without recognizing that the set was only tacked on afterward. Thus, don't be surprised if even if you can destroy a-z... the belief still stands. It was never based on a-z in the first place for most theists.

Lastly, to share my own tactics since I happen to be married to a believer... I never engage in any head-on confrontational debates anymore. I find it fruitless and that it only leads to dissent on both sides. It's difficult enough to discuss these matters... don't add resentment as a further barrier to someone listening to you and changing their mind.

I have found it helpful to simply ask questions. My wife just heard a talk on prayer and intercession and "Why god says 'no' sometimes." I found the whole thing riddled with issues, but I have come to find it more effective to simply ask inquisitive questions rather than ridicule or attack:

• Hmmm. Interesting. So, if I understand correctly, the speaker said that sometimes god wants to let us suffer to be more united with him. He also said we should pray only for things that are god's will. If that's the case, how might we know if someone gets hurt if we should pray for their healing or pray for them to remain injured to suffer and grow in holiness?

Things like that. After a number of those types of question (and my wife not being sure how to answer), I simply asked if she felt like the questions seemed to require somewhat convoluted answers. She said yes. Does't mean she's going to abandon belief. For now, she believes that someone "smarter than her" would be able to answer the questions... but I find it enough at the present moment for her to at least be brought to understand that the answers seem convoluted. And then I can introduce the fact that correct solutions tend to be the simplest ones that can account for all the details.

I'll depart with a flow-chart for these discussions I was just sent. It's filled with unstated assumptions and other flaws, but interesting nonetheless (LINK).