... don't get into the habit of joshing and joking and being sarcastic, because eventually the habit stays and it's difficult to change the behaviour. And then someone gets hurt.
When you're running/biking/jogging/walking/rowing/swimming and you want to stop, just tell yourself to get to the next lamppost (or tree, or recycling bin, whatever's nearby). Do this again and again, and you'll end up going much further than you thought you could
If it's viable, run/walk rather than taking the car to the gym. You'll do most of your cardio, so by the time you get to the gym you can focus on the less tiring bits of your workout AND you'll be so relieved you made it, you'll have positive associations of the gym!
Buy nice shower products, body lotions and lounge-pants, so that you can reward yourself with nice things after a workout. That way you'll have positive associations with working out.
When you work out (and only once you've warmed up), do your least favourite bit of your exercise plan FIRST. You'll have enough energy to get through it, and then you'll have sufficient endorphins to get you through the rest of the work out.
When you prepare a meal, cut up extra vegetables (celery sticks, carrots, cucumber, etc) and leave them in little zip-lock bags. Whenever you're hungry the following day, just grab a little bag. You won't be as tempted by the cake :)
When your alarm goes off in the morning for an early workout, throw your blanket/duvet/comforter off your body before you have a chance to think about it. That way, when your mind starts to argue to stay in bed, your body will have to get up because otherwise you'll get cold lying where you are.
Try smiling when you're exercising and you're losing breath/momentum: it releases endorphins and makes you go further/faster/better!
When you've had a 'bad' day, look in the mirror, make eye contact and smile (a PROPER smile) for 10 seconds. Our brains are wired to feel good when we see someone smiling. Oh, and smiling when you're exercising helps you keep going!