This has been discussed in numerous areas in the fora before, getting almost highly technical in some of the discussions but with no universal solutions proposed. Among the propose-able solutions are to restore the phone to factory defaults (especially if the phone is relatively "old" and cluttered), uninstalling unused apps, clearing old photos from the phone and clearing temporary files from the phone. Some of them work for others, while some won't work at all for others.
Let's all hope that this will be resolved completely in Windows 10 for phones.