Regarding motivation I have to agree with PP and that its about getting out there and just doing it. Plan a time that is convenient to exercise and do it then. Think of it like going to work. Get all your stuff ready in advance if you need to. Its much easier getting ready for exercise if it only takes 2 mins to get ready. If your mind starts with I should be exercising, but it will take x amount of time to sort my stuff etc then thats a demotivator and you are looking for excuses. If you have already said I will go for a run (for example) on a certain day and time then that is planned and just think of it as something you were always going to do.
I always use the mantra that I am more likely to regret not doing something than doing it. Someone did say to me one that you will never regret a run. Once I am in my gear I feel more motivated than before. I have had times when I really wasn't in the frame of mind for exercise and just could easily sit down and not bother, but as soon as Im in my gear something shifts that and its away I go.
Another big secret (as has been discussed) is finding something you enjoy. Some people (like me) enjoy running, some people (like my DH) hates it. There is also Swimming, Cycling, walking, rambling. A wide variety of classes such as Pilates and Yoga to Insanity and Kickboxing plus a huge range inbetween. The gym has lots of different things to consider. Treadmills, cross trainers, bikes, rowing machine, free weights, weight machines, kettlebells. Then there is also the vast amount of DVDs like Jillian Michaels or online there are things like insanity. Not forgetting about team/club sports if that is your thing. Netball, football, tennis, I know someone who plays in a female American Football team. Also Roller Derby is getting bigger I believe. Dont get demotivated if you find something difficult or not for you, just try something else.
Also it takes 30 days (I think) to develop a habit so just by starting off small (say 2/3 short sessions a week) and sticking to it for a month then it should get easier to get the motivation and perhaps then if possible make the sessions longer or add more. Although, having saying that, I exercise well for a few weeks then kind of go through periods of being lazy and not bothering.
Sorry for rambling I was on a role thinking about ways to help with the motivational side of exercising.