The hard work begins after you've landed an interview and are sitting in front of a potential employer. When I suggested that you enter armed with a relatively honest answer about why you want the job you're interviewing for, syako asked for advice on how to respond when you're only planning on staying at a job for a short while. To see my advice, read more.
Savvy says: Approach the question as if you don't plan on leaving any time soon. While right now you may view the position as a stepping stone before you move on to something bigger and better, there's no telling how the job you were hired for could change in the time you're there. Instead of channeling your long-term career plan and laying it all out for your interviewer, stay mostly in the present while indicating that you hope to grow with the company.
Consider what attracted you to the job opening in the first place. If it's a lateral move, then you'll want to talk up how much you want to be a part of the specific company and how you are excited about the possibility of bringing your developed skills to the job while exploring new ones as called for by any subtle differences in job descriptions. Reinforce the things that you like about the job — problem solving, working with people, being kept on your toes, etc.
When it comes to bringing up the future, instead of giving off an 'I'm using you until something better comes along vibe,' simply discuss how the particular job will help your growth in the meantime and that you're certain your current skills will make you a fast learner.