It might be hard to imagine, but someday you will have a life after the military. Early thought, careful planning, and ongoing preparation will help make the transition easier. In my opinion, it is impossible to over-plan for such a huge change to nearly every aspect of your life. There are all sorts of issues with leaving the military, but I'll stick to financial because that is what I know.
Just like any other financial planning, there are two basic parts: the money that will be coming in, and the money that will be going out. Both are hard to estimate. I've just spent the better part of two days doing hypotheses about when my husband might retire, and how much his retirement income might be, plus our possible expenses and variables such as second income and children still at home. There are almost too many unknowns to even start thinking about it. Don't be intimidated! Just writing what you sort-of-know down on paper is a great first step!
There are also some universally good things that everyone should do:
1. Build an emergency fund. Experts say 6 to 9 to 12 months expenses. I say, save as much as you can. Even $1000 is much better than nothing.
2. Pay off debt. You need to leave the military with as few financial obligations as possible. Credit cards and high interest loans absolutely should go, then hopefully you can pay off car loans. Make those monthly bills go away!
3. Learn about your benefits and the resources available to you. You might have Tricare coverage for a period of time, or you might be transitioning to Tricare's retiree program. Knowing the coverage, costs and details is very useful. Are you eligible for the GI Bill, and might you use it? BAH is a huge benefit to the Post 9/11 GI Bill. If you plan to use the GI Bill, or your spouse plans to use it, then factor BAH into your calculation. However, don't forget that it is only while you are actively in classes.
4. Sharpen your skills. Even if you have been in the military for 30 years, and you don't anticipate returning to the workforce, skills are always good. Take advantage of tuition assistance while you are still in the service. In addition to your education, see if this is the right time for your spouse to learn some new skills as well.
5. If you are married, spend a ridiculous amount of time talking to your spouse about retirement and all the various things that will change. My poor husband must be sick of me accosting him with questions like, "Do you plan to return to school when you retire?" and, "What do you want to be when you grow up?" However, it helps us both. I understand his hopes and dreams for the future, and he understands the economics of the situation. Of course, your role in might differ in your relationship, but communication is good.
Even if retirement or ETS seems forever in the future, you will still find yourself surprised as milestones pass and the time becomes nearer. Milestones are useful to keep yourself in line, but they aren't carved in stone and they will vary depending on your specific situation. In general, however, earlier and more is way better than later and less preparation. I was recently speaking with a Transition Assistance Program (TAP) manager at a large Navy Fleet and Family Service Center (FFSC) about the resources available to service members retiring or leaving the military. He stated that most people were only meeting the minimum required use of the TAP resources, and that he actually recommends that people attend the TAP class earlier and more often than the guidelines recommend. While your command doesn't have to approve of this, and your FFSC may be too full to accommodate you, it is worth a try.
No matter where you are in your military service, give some thought to what your life will look like after you leave the service. Every hour of preparation now will make the future so much easier and more pleasant for everyone involved.