After our first we definitely wanted to have another and we knew we wanted them close in age. My husband was supposed to deploy in April/May of 2008, and since I did not want to give birth while he was gone we made sure that July of 2007 would be the last month we would 'try' so that he would still be here when the baby was born, otherwise we would wait until he returned home. I got pregnant that July and he switched units and didn't deploy after all so it worked out.
After we had #2, I didn't want to do any permanent birth control 'just in case' and he agreed but as time went on was good with just the two and said more and more "we're done"... and so was I but of course would always be happy if we ended up with a third.
#3 was a surprise! OOPS! And having to tell him was awkward since I new he wasn't crazy about the idea of having more. (and it had to be over the phone while he was gone for 6 weeks). After the initial shock wore off, he was totally on board with it and we're both excited!
I think being a military family makes it more difficult because not only do you have to decide on how many children you want but you also have to figure out the 'when'...you have to do a lot more planning when trying to work around deployments.
I think you "know" when you are done...I was back and forth after #2 but now, so close to having #3 I know for sure that this is our family, this is what it was meant to be and we are not having any more.