A combination of pine bark fines, peat, and perlite works very well. Ideally, the bark should be aged so that it contains a wide spectrum of particle sizes (from very fine up to a max of 1/2" or so). For smaller pots, I've had good results with ratios from 6-2.5-1.5 (bark/peat/perlite) to 7-1.5-1.5.