I've been riding fixed for about a year now (1500 miles). I had a bike built up with a flip-flop hub, thinking that I'd need the freewheel as a crutch. I rode the bike out of the shop fixed and haven't flopped to the freewheel yet. For gearing I pretty much guessed, using the gear ratio I thought appropriate for the undulating terrain I ride the most, with a high cadence preferred over a slower 'mashing' cadence. The higher cadence forces a smoother cadence over time. I've since gone to higher gearing to build a little more strength while preserving the smooth, higher cadence.

I'd actually recommend starting on some nice flat pavement instead of a soft field and it doesn't take long to get used to riding fixed. Corners need to be taken a bit wide and with less lean to avoid pedal strike. "Don't stop pedaling" is a good mantra and especially good to keep in mind when approaching bumps and railroad crossings. Riding dirt paths with skinny tires on a fixed bike does wonders for bike handling. No matter what the path throws at you, you have no option but to keep riding.

Our last snowstorm and cold snap was enough to get me to put on the knobby cyclocross tires. The feeling of control with the more direct connection through the drivetrain is amazing.