My DH and I can't even agree how we came up with DS1's name! He says it was his idea but I think my colleague suggested it originally.
But basically we found out the sex, which cut down the arguments a lot! Then we both separately thought about names (looked on threads, name books, Office for National Statistics spreadsheets etc) and threw them at each other until one stuck.
We changed our minds about our original choice at about 6 months, when I found i kept meeting people who'd just called their DS that name, and thought it was becoming too popular for us to use.
We really only had one name (plus one back-up) we could agree on, so that was that. DS2 on his way and so far we're planning on using the "back-up" name we had for DS1 as we've not found any other names we can agree on. No middle names, thankfully, or we'd be spending the entire pregnancy arguing about names!
Both agree it has to be a name that works for an adult man as well as a child, unusual but not too weird, and able to be shouted across a playground without embarrassment.
Good luck 