Sit down and take slow breaths, do NOT lie down as this can affect your breathing.
Take more long deep breaths for 30-60 seconds.
Stay calm as this will make breathing easier and if possible, ask someone to find your inhaler.
Get away from the possible cause of the attack, this can be dust, cigarette smoke or the smell of chemicals.
If breathing is still difficult, have a hot drink like coffee as this will help to open your airways slightly.
If the coughing and breathing is still difficult, call 999 and ask for an ambulance.