1) Make your family do it with you.
2) Don't talk about it, do it.
3) See #2.
Seriously, I think everyone knows how to do it. Less calories in, more calories out. One of the dangers of asking general questions... especially motivation related... is that you aren't going to get a specific plan to stick to. That never works.
Asking for specific advice, like "I have only about 30min a day to exercise and need to burn roughly 700 calories" can be answered and guide you to achieving the goal. Same with diet - "I want to eat 1900 calories but ensure I keep myself balanced nutritionally" can be answered.
What can't be answered is "I like to eat chocolate at 3:00 AM, how can I stop doing this?"... Well, except for saying that you shouldn't have chocolate in the house. Anyway, the point is that you can break your motivation and nothing you do now will change that. Almost everyone knows what needs to be done to achieve it - it's the willingness to do it that is lacking. Tired after work? Get up earlier and excersize then. Stuff like that.
(BTW, the answers to those two questions are "Elliptical machine at roughly 6 miles per hour at a mild to moderate setting for 30 minutes" and "Get a damn planner and put in the time".
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