I say/rmind myself "I can do this!" I say this to myself as I often doubt I can carry on or can even begin what I want myself to achieve.
Another thing I do is to say to myself to do "just the first 2 minutes" then I can change my mind if I want. After those initial 2 minutes I feel a bit better, then I ask myself if I want to carry on, if its aches and pains related I try to get to 5 minutes, ask again, if I feel a bit better I go to 15 minutes, if ALL the aches and pains haven't subsided I can then stop.
If its boredom or something else stopping me complete I focus on "how amazing will it feel if I run for XX". I managed an hour and a half on a day I hadn't even anticipated running for when I did that!