Listen to everyone who suggested you should ask questions. That's your best shot. Witnesses don't like being preached to. (who does?) But they're accustomed to people asking them questions.
You could have the most meticulously researched, airtight explanation for the generation, false prophecies, blood, 607, etc., but it won't matter because as soon as you start preaching the JW will shut down.
You'll have much better results by simply asking questions and, if the conversation goes well, perhaps gently introducing some facts (for example, "I did some research in the library and was surprised to see that none of the encyclopedias say Jerusalem was destroyed in 607" or "I read in the Proclaimers book that for decades we taught that the Last Days began in 1874").
You probably know this already, but make sure you have non-apostate sources for any information you plan to present. Obviously the best sources are WT publications, but scholarly publications like encyclopedias are ok. Witnesses will often dismiss information coming from websites/blogs (even if they're not "apostate" per se).
Be prepared to have your motives questioned and for the JW to ask where you got this information and why you're asking these questions. Witnesses are trained to be scared of this type of questioning. There's not much you can do about it other than be prepared for it. A good scripture to have in your back pocket is 1 John 4:1 which says not to believe every inspired expression but to test it. If you get heat for asking uncomfortable questions, you can point out that the Bible commands Christians to test what we hear and read--and that's what you're trying to do.
Don't try to hit it out of the park on the first conversation. Be patient. If JW stuff doesn't come up during the conversation, don't force it. Focus on keeping your relationship with your friend first. If religion does come up, play it cool. This is hard to do, I know. If you see the walls start coming up, you should be the one to change the subject to something other than JW/WTS stuff. Don't be the stereotypical obsessive-compulsive "apostate."
My final piece of advice is to be honest. This does not mean you should tell your JW friend everything. But it does mean you shouldn't lie, exaggerate, or overstate your position (for example, I find that some ex-JWs overstate issues related to what the WTS published about 1975 and I think it hurts their credibility). If you are wrong on any fact, the JW will seize on it and use it to discredit everything else you say. There is plenty of well-documented material, so there's no need to cut corners or exaggerate.
Hope this helps.
