With CO2 just use minimal pressure to fill the void and keep a little pressure. I jsut top up manually when required. Others use 1 psi from the regulator.
The way most UK pubs do it if they are using a keg or don't get through the Ale in a day or so is to use a keg breather. It just allows co2 to enter as the beer comes out.
Well done for going the "real Ale" route
Here's to flat(almost) Ale.
The only way with out will allow air in so you need to finish the brew with in a few days.