While there's no proven 'boiler plate' method on how this delicate subject should be approached, here are some common denominators that many experts agree on as the most pain-free way to broach this sensitive subject.
Many clients I've spoken to tend to put off talking to their spouse about this topic because they feel uncertain on how to best approach it. I recommend a common sense approach, though many clients ask the valid question, what constitutes common sense?
The Golden Rule of treating others the way you'd want to be treated is a key point to keep in mind. How would you want to be approached about this subject if the tables were turned?
Choose your time wisely
I think anyone would agree that when your spouse has come home from a long day at work, or when they are tired or hungry aren't the best times to discuss any subject of importance. Some of my clients have even found that a successful action is to 'set up an appointment time for a discussion'. This can be done by simply getting your Spouse's agreement that there's an important subject that you'd like to discuss and deciding on a time that works well for both of you to go over this.
A key to your success is also to make sure there aren't any distractions when you have this discussion:
Make sure the kids aren't around, turn off the TV, phone etc. If you've planned ahead, you should also have allowed plenty of time for this conversation. When your spouse is on the way out to a meeting or work, obviously wouldn't be a good time to say, "By the way, I've been thinking about a divorce".
Try not to let it deteriorate into an anger match
I know this is a difficult concept and obviously the conversation is going to get heated but if it degenerates in to a 'slanging match' of accusations, not much will be accomplished and you may both end up saying things in anger that you later regret. It's okay to walk away and take up this discussion later when cooler heads prevail.
More communication not less is always the answer. It is possible to have an amicable divorce. It doesn't have to turn into World War 3. This is especially important to consider when there are kids involved. Isn't it worth making the extra effort to minimize any potential upset that the kids may experience when Mom & Dad suddenly turn into sworn enemies?