As GTC has stated, setting achievable but challenging goals is key. Sometimes it's more motivating to aim to run to a specific location, and not being concerned with the distance or the time. You may also want to take a look at the NHS' C25K plan.
My biggest motivation for running comes from Dean Karnazes' book, The Ultramarathon Man: Confessions of an all-night Runner. Give it a read, highly inspiring and will help tackle any motivational issues.