A lot of this is incorrect and based on common misconception. Carrying a balance on a card does not necessarily help your credit score. Good payment history should be shown on school loans, car loans, or mortgages. Extending out payments on a debt you could pay off is stupid. Closing accounts, especially old ones, will definitely hurt your score. The amount of available debt potential is not as important as your debt utilization. Under 10% is ideal. Having 50k available to you would put most people in a better score bracket as it would lower their overall debt utilization ratio.
Yes, the 3 credit ranking bureaus use complex algorithms that are not completely understood, but taking the above poster's advice will surely have a negative impact.
On closing accounts