Tell yourself that you're going to sketch for at least 10 minutes every day in that book if you're not so used to keeping consistent and motivated every day. Me, I have trouble not getting carried away and making the sketch into a complete, finished, perfect, etc. drawing/painting (and then every time I look at the sketch book, I get nervous about what's going to go in it next, which is not what you want to happen)...so when I realize that I'm paying way too much attention to detail and correctness, I remind myself that no one has to see this, that this is supposed to be making me better and more circumspect about style and technique, that I'll feel more comfortable the more zen I am about drawing...it's as if you have to feel safe and not safe at the same time when sketching, like walking on the edge, in order to improve and for sketching to have purpose.
Set a rule and stick to it. For example if your goal is to sketch at least one page a day (which is a very reasonable one), you do not go to sleep until you have made that page. That's it. If you have a couple of hours free time and it's a nice day, you go out for a walk with your sketchbook and you do not go back home until you've made a sketch of something in it. Try it, it builds character.
I always carry my sketchbook everywhere and I try hard to stick to "a sketch a day" rule.
If you are having a tough time getting motivated to sketch every day I'd recommend setting a really easy goal to start with. Before I start working, I like to warm-up with a few 30 second sketches. If you decided to draw five 30 second poses in your sketchbook everyday you would start filling those pages quickly. Plus, if your goal only takes a couple minutes each day how can you avoid it? Hope that helps
There's no wrong way to use a sketchbook, it's usually personal to each artist. I've got two sketchbooks on the go right now:
The first one is 8x10 sized and I use it for general purposes - brainstorming, initial sketches, thumbnails, studies, works in dry media like coloured pencil, etc. Most of the finished artwork I do starts life here.
The second is a small purse-sized watercolour sketchbook I use only for drawings from life. Whenever I can't think of what to draw, I'll sit in front of something and draw it here. I've done landscapes at a park, various views of the interior of my house, still life with things like my tea pot, my pencil case, etc. I've got a few watercolour pencils and a waterbrush I like to use in here.