I will not try to attack their faith. (i will let them do it themselves)
One thing to keep in mind. Jw will typically "bait" you into attack mode. During a conversation negative aspects of the JW faith will be dismissed, pushing you further and further towards frustration. In an effort to overcome their dismissive attitude, you may be inclined to exaggerate things in hopes of getting your point across (e.g. focusing on very contriversial subjects like blood and DF). The instant you show any emotion they will say "got ya!", believing that you are just angry and defiant and not really in search of "truth". You are "full of pride" and deserve little respect.
I only had a few conversations with JWs that I would call productive. In each case the JW agreed with me when we talked, and showed very little of a defensive attitude. In every case the JW went back to JW Elders who convinced them that their doubts were unjustified.
When I had these conversations I had an extrememly structured presentation. The JW was given very little time for rebuttles and argument. During one conversation, I had photocopies, bible verses, and organized documentation. I went point by point, asking that the person consider the Watchtowers own information. For over two hours I spoke with an Elder who eventually said that I was right. He appeared very scared (my wife actually was affraid he might be very distressed at seeing that it may not be the truth). The truths I confronted him with eventually made him angry at me, making our JC situation worse. Because of a group mentality, the Elder ganged with two other elders and did a complete 180, acting harshly towards us during the committee meetings. Yet, he visited my house once more and displayed the same kind attitude as he did when the other Elders wern't around. Kind of like a dual personality.
If you dad is actually open to listening, I would suggest you have everything you want to present documents. Lots of websites have the quotes organized that you need. Make sure it is current. Don't just go off saying "you know that they do this....". The JW will simply deny it. Use the publications, they are the authoritative source.