Sometimes by lonely tone, sometimes by humor.  I started by picking Army because of my BIL who served. But then I 'branched out" just because some of the posts spoke to me.  Sometimes I pick by location, trying to reach Kuwait and Qatar and Afgh. Or if they sound like my sons (age 20), I'm a sucker.  By and large, tho', it's by the ones with the fewest requests relative to the unit numbers.

It is hard to decide much of the time.  But I found that somehow, like many have already said, that one person just speaks to you.  Before the contact count came out I used to pick by where their location was, if I could tell they were on the outskirts away from any base I would pick them first, I also look for people from the Marines because of my brother who was a career man.  But then I would think of the guys in Afganistan and wonder if they felt forgotten so I seek them out too. 

I developed a relationship with an airman who flew helicopters in Afganistan, his letter said he was going to be taking packages to the guys in the mountains away from the base or any comforts.  He immediately caught my eye because he talked about his fellow brothers that didn't have it as good as him.  I worked with him until his tour was up.  I miss "talking" with him.

I guess the best way to pick a contact is follow your heart, no matter who you pick, they wouldn't have signed up if THEIR heart wasn't in the right place too.