There's good advice here from people, especially the personal examples of how the assignment officer's attitude changes when you reach 14-16 years of service (and there are fewer billets for your higher rank). The closer you get to 20, the more limited the choices and the faster the fun stops.
And, yeah, I should've seen this next one coming. I'm getting PMs and e-mails with questions about the details of calculating a Reserve/Guard pension. Here's a generic answer, and you can insert your projected rank into the pay tables.
Keep in mind that this answer is also generic at the DoD level, but your service will have more specific requirements (especially for "good years" and points) at the unit level.
The Reserve/Guard retirement is based on points instead of years. When you leave active duty for the Reserves, you'll start your point count with one point for every day of active duty (365 or 366 points per year) and with credit for a certain number of "good years" that you earned on active duty. Your goal is to rack up at a total of at least 20 good years (active + Reserve) with as many total points as possible.
A good year is awarded for completing a minimum number of drill weekends (generally 10) and for earning a minimum number of points (generally 50). A good year also means that you've complied with unit requirements for mandatory training, bodyfat composition, physical fitness tests, medical/dental readiness, and other details. Once you get to 20 good years you can file for "retired awaiting pay", or you can keep on drilling to 30 good years.
Some years you might do more duty for more points, and maybe every five years you'll mobilize for 6-12 months (one point per day of mobilization). You could conservatively estimate your Reserve points to be 50 points per good year. A realistic estimate might be 60-70 points per year.
Your pension will be:
Monthly pension = [points / 360] x 2.5% x High-Three pay base. (The divisor is 360 because military pension months only have 30 days.)
Your High-Three pay base is complicated. It's the average of the 36 months of highest pay, of course, but when you retire awaiting pay then the calculation will include all the pay tables up until the day you reach age 60. It'll also include your longevity as though you'd been on active duty the entire time up until age 60. If you joined at age 29 then at age 60 you'd be assumed to have 31 years of service for pay purposes, or the >30 column of the pay tables. (It's actually the High-Three average of one year of >28 and two years of >30, but that's the same amount of pay.) Admittedly nobody knows what will happen to pay tables over the next 27 years.
http://www.dfas.mil/dam/jcr:7061e0ca-a436-42f9-aa30-1a93b6454aa3/2015MilitaryPayChart.pdfThe pay tables will presumably keep up with inflation over the next 27 years, so you could keep track of your Reserve pension in today's dollars. (But you can see how these assumptions start to pile up over nearly three decades.) If you had four years of active duty and 16 good years at 50 points per year, then in today's dollars your low estimate of your Reserve pension (at age 60) would be:
[((365x4) + (16x50)) / 360] x 2.5% x $(High Three average)/month = $1375/month.
This is a low estimate because most Reservists accumulate 2500-4500 points. However the pension is an inflation-adjusted annuity at age 60 with a life expectancy of another 25-30 years, so a smaller Reserve pension will still eliminate the chance of portfolio failure. You'll also have Tricare and Medicare so your health expenses will be very low as you age.
Here's two facts (with no assumptions!) that I've heard from hundreds of veterans in their 50s or older. Every freakin' one of them is either very glad that they kept drilling for the Reserve/Guard pension, or they're very very sorry that they didn't make the time to get the pension.
More details are at these posts:
http://the-military-guide.com/2012/06/02/calculating-a-reserve-retirement/ (the blog's #1 post for over three years)
http://the-military-guide.com/2010/12/06/retiring-from-the-reserves-and-national-guard/http://the-military-guide.com/2014/11/01/comparing-an-e-7-active-duty-pension-to-an-e-7-reserve-pension/Again, if you're still having fun then you should stay on active duty. But if the fun stops (or if your priorities change) then this is how you salvage enough pension to support your financial independence.