First, make sure you know when their deadlines are so you are not calling them at deadline time. The key is to be persistent. Email them with a friendly follow up after two weeks to ask if they got your initial email and if they want your work. If you’re pitching for a daily or weekly publication, reply sooner and ask them. You might want to say “please let me know if it is not a right fit so I can see if another publication may want it, as it is a timely piece.”  You might get rejected, but remember editors will be more likely to remember your name if you have recently pitched to them. You could unexpectedly receive an email commissioning you for something.