I get more and more experience in converting .osm into .img using mkgmap. What I see on my Garmin unit is, among many other useful things, a bunch of white lines: boundaries. I do not need them, apart from national boundaries. How to leave them out with mkgmap? I can’t find the proper option.
There a many possible options to attack this problem, from adding a TYP file, adapting the stylesheet to ignore the boundaries or preprocessing the data with e.g. Osmosis to remove the boundaries before feeding the data to Mkgmap.
Thank you. Nice home work to do 
best way: don’t process these lines at all. that simply means deleting the appropriate code from the style.
example from the lines file:
#this would process national borders, but no other borders
boundary=national | (boundary=administrative & admin_level=2) [0x1e resolution 16])
leave out/delete all other boundary=administrative lines.
for more complex problems, have a look at the delete command, here: http://wiki.openstreetmap.org/wiki/Mkgmap/help/style_rules#delete