Credit Score Management

I’m trying to boost my credit score as much as possible. I have credit cards but find it harder to get loans due to my score. I need help figuring out a sure-fire way to gain points by using the credit that is already available.

I have several credit cards that I have’t used in a long time. Should I close these accounts or leave them open? How will it affect my credit score?

I heard having too many credit cards can lower your FICO score even if you pay them full each month. How many can you get before they drop your score?
I had problem getting a credit card when I had no credit history and then after I finally got one, I had hard time getting another one, because banks said I had “too few” revolving accounts. Now I have 5 credit cards and only 1 of them carry balance. My balance to credit ratio is 6% and my FICO score is 740.

Okay I have two credit cards at 500 limit and last year I had to have some medical attention which caused me to use both credit cards I finished up using 85 percent of both of them and by that it cause my credit score to go down from 680 to 550 Now my question is I’m paying them both off this month and i want to know around how long before the credit company finds out and raises my score?

What happens to my credit score when I apply for a few credit cards and are not approved because they require brilliant credit? Does it affect my ability to get approval on another credit card? How long is this for?