I recommend starting with the following settings as a baseline, and increasing $fs if the GUI becomes slow.

$fn=.01;
$fs=.3;

Background

Three variables determine the faceted-ness of your model.

• $fa is the minimum angle for a fragment. It creates a size-independent upper bound for number of facets in a circle. This bound is exactly ceil(360/$fa)
• $fs sets the minimum facet size in millimeters.
• $fn is a complete override on the number of facts. This can be used when an exact number of facets is desired.
  • $fn is useful for creating regular n-gons

Old Method

I used to use different quality levels for preview and render with the code snippet.

$fa=.01;
$fs=$preview ? 5 : .1;

This allowed me to edit at interactive rates but render at the highest resolution visible on my printer. I no longer recommend this, because it can lead to unexpectedly long render times. Depending on the model, halving the minimum facet size can increase the rendering time 16-fold!

Better Method

I now prefer to set a single facet size of .3mm or 1mm. If the $preview variable is not used to modify facet size, then preview calculation time more accurately approximates the render calculation time.

More info on facets can be found at the OpenScad Wikibook