For me, the key to being motivated to blog is to keep it enjoyable. I want my blog to be something that I enjoy doing, not a hassle or a chore. I figure if I'm enjoying creating the content, it's probably more likely that readers will enjoy it too.
I'm not the kind of blogger who has every post planned out for the next two weeks. I often don't know what I'm going to post the next day. I'm not sure if that's a good thing... but so far I have never been able to come up with a system that works for scheduling things out more than a week in advance.
Create regular features that you enjoy writing. Things I love Thursday, Friday Wishlist, Weekly Wrap-up, etc. I like to do Q&A posts once a week and Remix Archives posts on the weekends, and then fill in with outfit posts, recipes, and other posts. Some people have every day of the week scheduled with a specific type of post.
Read other blogs that inspire you. I enjoy reading other blogs that inspire me to be a better blogger. Whether it's great features, improving my photography, getting better at writing, etc, reading other blogs that are high-quality can inspire you to get better at blogging!
Find something you enjoy posting. Don't blog things just because everyone else is doing it. If you don't like doing outfit posts, you don't have to do them! There's no rule out there stating you have to post certain types of things. The blogosphere is full of readers with an infinite amount of varying interests. You're a totally unique person, so let that shine through in your blogging. You'll enjoy blogging more if you're being yourself, rather than trying to be someone else just because they're a "big successful blogger."