Include the contact details of charity employees who are available for comment in the Notes for Editors section of the press release (including a mobile number, if possible).
Daily newspapers, TV or radio will usually make contact within a few hours if they are interested. Weekly newspapers may not contact you for a week, so make sure you haven't got any holiday booked off soon after the release goes out.
If a journalist has emailed the charity or left a voicemail, respond to their enquiry as quickly as possible. If you can't give them an answer straight away, let them know you will look into it and tell them when you will get back to them.