Consulting is a job, and like any job, it comes with the possibility of becoming stale, uninteresting, or boring. This is especially true if you have a single skill set or are focusing on one particular skill. When a job becomes boring, you tend to start becoming disinterested in what you do and start looking to anything else but your work for enjoyment. Even if you love your job and love what you do, you can still get tired of doing it.
The trick here is to find ways to make it to keep things fresh and interesting. The best way to do this is to focus on something that falls outside of your normal realm of interest. If you are a programmer, try learning database design. If you are a database person you might want to consider looking at learning a programming language. You might notice that both of those examples mingle in real world work situations. As a programmer you will generally connect to a database at some point for some reason and a database person might have to design databases for applications. This gets those kinds of people used to thinking about things in relation to what they already do. This is a great way to maintain interest while staying relevant to what you already do.
The other approach is to take something completely foreign and dive into the deep in. If your primarily a programmer, jumping to project management is something that is (usually) completely different. There are many styles and techniques to project management so there is a lot to learn there. This generally requires the purchase of books and the attending of classes specific to the subject you’re trying to learn. The deeper you go and the more you learn about the new topic, the faster you can get onto projects where you can use the new skills. In the long term, this allows you to move onto different kinds of projects more frequently.
Also, finding other ways to occupy your time that are beneficial are good as well. Partaking in social networks related to your field, writing a blog, or conversing with others in your field can occupy your mind while keeping it in a relative mindset. The goal here is to not become bored for too long if you do. Staying bored is a good way to drain yourself and the longer you stay in that mindset, the longer it will take for you to get out of it.