What I did when I built my first fixed gear was ride my commute and around town every day for 3 or 4 days in one gear as much as possible. If I needed to shift down, made sure to remember that, if I needed to shift up spinning down a hill (granted, spinning on the fixed is way easier) made a note of that, too. From there it's just a matter of picking somewhere in the middle.

If it helps, I ride 46/16 in Pittsburgh, which is pretty hilly, so you end up pushing up the hills, but you don't spin out so quickly going down them.

Edit: But I started running a 44/16. I didn't switch to a 46t up front for probably 4 months of riding the fixed gear 125+ miles a week.