One of the best ways to stay motivated with your workouts is to train with a partner. Having a training partner will hold you accountable to show up to your workout in the first place, and a good training partner will also encourage you to challenge yourself during the workout.
This is one of the reasons why people can benefit from working with a personal trainer. Of course, learning to use correct technique, following an effective program, and choosing the most productive exercises are also very important reasons to hire a trainer. But I would argue that once you have learned the basics of using good form, and you understand the program you are following, then motivation becomes a much stronger factor. Working out with someone who won't allow you to "slack off", who reminds you of your goals, and who encourages you to train with greater intensity, will bring you much greater results than going it alone.
I was recently reminded of this as I have now started working with a "trainer" myself. Since our daughter was born I have been sleeping less and training less often. I have also been extremely busy with clients and other aspects of my business (working on producing a training manual and another DVD). Basically, those are my excuses for "slacking"! I had to admit to myself that I needed a kick in the pants, so I asked a friend and colleague to become my training partner. I insisted that he not accept any "excuses" from me, and I agreed not to cancel any of our workouts once we scheduled them. There was certainly some inertia to overcome, but we are both benefitting at present, and I'm starting to feel like myself again.
Some people may prefer to train alone, and some may already be extremely motivated and driven to succeed... that's great! But if at some point "life" is getting in the way, or you think you are too busy, then consider that it might be best to find a good training partner or hire a professional.