There's lots of different techniques to achieve the worn/chipped paint look. I'd probably give the entire helmet an aluminum or metal base coat for starters and then choose how many different layers of color you want to use? Chipped paint can be achieved with the following methods:
Rubber Cement: whichever base coat you have on there,you can randomly apply rubber cement(to fully cured paint).You can splatter it on there,or apply with a brush or spmething else.Spray your secon,third or whatever coat it is you are working with over it.ASfter it cured,peel the rubber cement off exposing the paint beneath.
Salt: Wet/dampen the dry/cured paint surface and coat with table salt.For larger chipped paint effect, you might consider using rock salt.Let the salt dry onto the surface before painting,then spry your new color over it.After the top coat drys thoroughly....brush off the salt from the model surface exposing the the coat underneath.
Future Floor Wax: basically a similar technique. Apply the wax in drops or spatter it on the dry model surface. After the wax drys, paint your new coat ove rit.After the new coat drys, remove the wax from the model surface with some double sided masking tape.Rip/tear the wax away exposing the color underneath.
Hand Brushing: The good 'ol steady hand brushing technique. This is probably more time consuming, and there is a fine line between hand paint chipped/worn paint look to surfaces and over doing it, but it works just fine.
There might be other techniques out there,I'm sure someone mentioned a different way of achieving a worn/chipped paint look above. Like they say, "there is more than one way to skin a cat."
