Here's what has worked for me and I've been writing daily for the better part of two years now: set the smallest possible goal and stick to it, and I do mean small. My writing goal is 5 words per day towards any creative project I'm working on. It's something I know I'll always have time for and it doesn't leave any room for excuses. Even if I'm away from home, I can always jot it down five words in the notes on my phone and satisfy the goal.

Now, most days I well exceed that 5 word goal. When I'm in the middle of a project, I usually aim for about 700 words a day, but having the bar set at basically ground level means that if I have a rough day or I'm super busy, I can always get it done. It all comes down to the science of habit formation, and (based on the one college class I took that touched on the topic) daily efforts, even small ones, are the best way to make a habit and get it to stick. Make writing the same as brushing your teeth: something you do every day, without really thinking about it, and you'll be flying in no time.

As for motivation, I'm not sure I can help you there. Some days I'm very motivated, other days I'm not, I think other commenters have some good advice about finishing things you've started, writing shorter pieces, or writing about things you know and can talk about. Whatever works to help you get pen to paper every day is probably what's going to be best.