Here are some tips to help you free up storage space on your device.
First, you should try and determine what is taking up most of the space on your device. To do this, open the iOS Settings app and then go to General > Usage > Manage Storage. Here you'll see how much storage you still have available on your device. You'll also see a list of apps, sorted by the amount of storage they are consuming.
IMPORTANT NOTE: Do not delete anything that you want to keep and do not have a backup. Do not delete photos or videos that you do not have copies. Make a phone backup to iTunes or iCloud before deleting anything.
The fastest way to get more storage space is to delete unwanted or unneeded:
1. Applications
2. Videos in the Photo app
3. Podcasts
4. Music
5. A large number of photos
6. Unused projects in Splice
There are some applications that will download, in the background, until they are done or until the phone is full. If they stop when the phone is full they will resume once you free up some space. For example if you do not set a limit for podcasts in the podcast app it can do this. Dropbox and iCloud have also done this in the past. Look for anything that downloads in the background and disable background downloading.