move at a steady pace too when practicing, not too slow... (but not so rapid you have gaps in the line) I'm no expert on thin lines myself but when I do get satisfactory ones it is by using more than normal thinned paint (1:1 that Scott mentioned is a good number), low pressure, not a whole lot of paint coming out of the nozzle (just enough to creat the line and not spread to the sides of it as I spray) and by moving at a decent enough speed as not to cause the line to widen from sitting in one place too long.
A lot of these paint amounts and speeds will only come with practice as Scott mentioned.
sorry if that sounds confusing!!!