If you're trying to build a business whilst you're still holding down a full-time job, then you must be super-vigilant about using the little time you have in the most productive manner possible. It's so easy to get distracted online and spend the precious hour or two you have to work on your business on Facebook or reading emails or on some other senseless task. So set a goal for yourself for each work session and stick to it. It helps to set a timer for 45 minute segments. Then work like crazy for 45 minutes on the task at hand. Take a 15 minute break, then set the timer for another 45 minutes and work again. It's vital that during your 45 minute segment you do nothing else. Don't check your email, answer the phone or do anything else. Just work.
If you do this for several weeks or months, you'll be amazed at the progress you make.
Also, make use of time that others waste. Rather than spend your weekends watching TV or wandering around the mall, use that time to work. You will have to make some sacrifices if you want to succeed at this business, but they will be so worth it.