I also have problems keeping motivated sometimes. It is very easy to take a few days off from your business and this could of course lead to big problems later on. It is important to schedule your day thoroughly.
Read a lot of success story. Reading those inspiring story make me realize, I'm not the one who struggle so hard for my business. A lot of people out there experience the same feelings. This help me so much to stay motivated.
The thing that keeps me motivated is the rumble in my belly! (only partly joking here) I love the feeling of depending upon myself and not the whims of some crazy boss (oh, wait my boss is still crazy...)