A Single Arm Dumbbell Press is an incredibly effective exercise for your shoulder stability - something that is surprisingly useful for running. This exercise also works your core and helps to prevent strength imbalances.
To set up for a Single Arm Dumbbell Press lie on your back on a padded bench with your feet planted into the floor off the end of the bench. Hold a dumbbell in one arm out to the side of your body with your elbow at the same level as your chest. You should then begin the movement by raising your arm towards the ceiling.
As you will be working just one side of your body you will find that you are lopsided. In order to counteract this you need to engage into your core to stop your body leaning to the side. Additionally, moving both your arms in the same movement (one arm without a weight) can help.