This is where the classic 1-second rule should be applied. Look
once for 1 second, then look away, and never look back again.
Note: However after this, because some women simply need to get
noticed (and obviously dress accordingly), if you are sensing that
she is giving you looks back (and possibly slowly getting pissed
because you are ignoring her), then (and only then) is it okay for
you to give her a second glance, and perhaps a well practised
curtious smile to brush off any awkwardness.
Dan