Ok, I am a bit of an exercise fanatic and feel quite down when I'm not on form. So here's a few tips from me:-
Write your exercise in a diary like appointments so that you have a scheduled time. That way you are less likely to find other things more important to do.
If you are running short of time that is not an excuse to skip exercise, you can get a decent workout in just 15 mins so you don't need an hour or more.
Make plans with people, that way you are less likely to give up or cop out. Classes with friends are great, especially if you are competitive and want to do better than them
Team sports - the personal development aspect is great plus you get a social side too. Also if you get involved in team games you are letting other people down if you don't attend.
If you are feeling very unfit start with something gentle like Pilates or yoga, if your not into series classes things like Zumba, pole dancing etc can all be fun ways of keeping it
Sign up for a challenge so you have something to train for. This year I did the London to Brighon bike ride for charity having never really cycled before. I'm currently deciding on my next challenge.
Finally at weekends get out and do active things with the kids. Some stuff we enjoy as a family are bowling, cycling, canoeing and ice skating.
DVDs I've enjoyed are anything Davina, anything jillion eg 30 day shred etc. nell mcandrew, billy blanks ate no (old but good)
As an idea I use my lunch hour to do a 30-40 min workout at least 3-4 times per week which are usually cycling, running, circuit training or gym resistance work outs. I also do an hour most evenings which is made up of boxercise, Pilates, netball matches or training, etc
Hope some of this s helpful for you x