I'd be careful listening to the "don't apply for new credit" advice. That may or may not be wise advice; depending on the circumstances, "don't apply for new credit" can actually be a canard.
The credit scoring system is inertia-driven...the more positive "mass" you have (TLs in good standing, preferably with higher CLs and longer ages), the higher your score will be, over time. If you're coming out of Credit Hell and you have no positive TLs, you definitely want to apply for credit, because getting a positive TL is the only way your score will go up. If you only have a couple positive TLs, and you haven't gotten a credit card in the past three months, I would apply for another one as long as you have a realistic chance of getting it and the annual fees are reasonable given your credit history.
The time you definitely don't want to apply for new credit is when you already have several positive TLs and you're looking to get a mortgage in the next three to six months.
