There is not really any rule of thumb per say about how to use creatine. But I make sure to take it when I am in a heavy training phase (i.e. if I were preparing for a powerlifting meet) to help maximize my strength gains. I also make sure to take it when I am dieting for fat loss (i.e. if I were preparing for a bodybuilding contest) because taking creatine while dieting will help to maintain lean muscle and keep your strength up on a calorie restricted diet.
But you can't train all out for ever. You do need to ease off the intensity at times to give your body a break. It is during these phases when I am not training for competition and really pushing myself extra hard in the gym that I will take a break from creatine supplements.
Even if you are not a competitor you can still make up your own training cycles based on your fitness goals. For example, right around the holiday times a lot of people will back off a little in there training. And focus on other priorities in life such as spending time with family and friends, etc... During these times like these when you are not really pushing yourself in the gym is a good time to take a break from the extra supplements such as creatine.
So you have to use your own judgment on this one based on your training. If you are are going to buckle down and get super strict with your diet, training, etc. and focus on a particular training goal for a few months then this is the time to take creatine to help maximize your training efforts. If you know that you won't be able to be 100% committed to super strict training and eating for a while for what ever reasons (i.e. extra demands at work, school, home, etc.) then there is no need to take creatine. Basically, what I am trying to say is only take creatine when you need it :-)
You can mix a teaspoonful of pure creatine powder with a protein drink, glass of water, juice, etc. I mix mine with my protein drinks. I usually have 3 protein drinks per day and I will mix a teaspoonful of creatine with each of my protein drinks. I will have a protein drink in the morning with breakfast, another in the afternoon, and another in the evening. On the days that I workout I will always have one of those protein drinks right after my training.