What tips do you have for successful “win interviews” and who is typically involved?

You're trying to figure out why they bought your product, what materials and conversations were most useful in their process, and setting them up for a case study down the line. Questions should go in line with those, and the interview should usually take under an hour unless they're talking your ear off. Dig deep for things that can be improved. Don't bring sales along for the ride!

One other point to add on to Derek's excellent advice. When setting them up for a case study down the line, I'd also encourage the customer to start thinking about/tracking any relevant metrics. Many companies, especially smaller ones, likely won't be very good at tracking results. So by talking to them about business results from the beginning you increase the likelihood that they might start to pay attention to those earlier rather than later. giving you the advantage of more compelling case studies with statistics

I do win interviews alone, but often have a warm up call with them and our sales rep for introductions. I've found this helps ease some anxiety from the sales side (some are nervous you may derail future sales/up sells, etc), but also presents a nice, warm hand off.

Be inquisitive, dig for the 'story'. Use open-ended questions. 

I agree with Derek. No salespeople. Keep them far away or you will get mixed results. Salespeople can be helpful with providing and intro and (maybe) giving some background.

For clarity, interviewing a sales rep is not a W/L interview. 

I typically do them by myself without the sales person or product mgmt. I find the customer is more open about product and sales process that way pm and sales contribute to info we want to gather though

I prefer to have a two-person team to do interviews - one person to conduct the interview, the other to take in-depth notes and identify follow up questions. This can be less critical if it's possible to record the interview, but I find that the written notes are fantastic for sharing the takeaways with others afterwards.

I found this post pretty thorough in terms of a process, questions, etc: https://labs.openviewpartners.com/how-to-conduct-win-loss-analysis/#.W2OIs9hKjVq

I referenced it when writing about a broader win / loss analysis process: https://www.crayon.co/blog/how-to-do-win-loss-analysis-examples-resources

Oftentimes, "win / loss analysis" gets equated with prospect interviews but it's just as important to measure the overall trends - both for tracking changes over time but also for identifying areas you want to dig into during the interview process. For example, you may see that a common loss reason is "budget", so you can dive into budget questions in your interviews with both won and lost opportunities.

Consider creating a pre-interview online survey and sharing with the interviewee prior to the interview. Done well, this will help you collect a ton of structured data over time so that you can conduct trends analysis. Their response to the survey will help you quickly identify where to probe during the interview. As an example, they may have selected you but scored the salesperson very low in a number of areas. 

Build an interview guide so that you're consistenly asking the same general questions interview to interview. This will help you identify trends over time. It's just a guide so be prepared to go off script to probe into areas of interest.

The interview guide should include key sections such as business drivers, selection criteria, influencers, product, price, competition, sales. 

Each question within the sections should be clear and concise (to avoid confusion and wasted time) as well as open-ended.

Ask for permission to record the interviews and have them transcribed (we us Rev.com). This will allow you to go back and listen to verify what you heard. 

Combine the transcript with your interview guide to generate the Q& section of your report. From there, write a short summary for each section to create your key findings/executive summary. 

Favorite question, "If tomorrow, you were the sales reps boss, what advice would you provide to them regarding improvement opportunities?"