a) Closing all old accounts improves your credit score. This does more damage than good. It shortens your credit history hence leaving you with a smaller amount available.
b) Opening more credit card accounts improves your credit score. You will come off as an irresponsible person financially. Lenders know that someone who has great financial discipline does not need too many credit cards to get by.
c) Paying off delinquencies restores credit. This gives you some points but the collection accounts, charge-offs, and late payments will still reflect on the account. You can have the collection agency remove the delinquency once you pay off the account.
d) Making a payment before the due date helps your credit score. This is not true, but if you pay off prior to the statement closing date the report will show a zero balance for the account. It is important for your utilization rate.
e) All delinquencies are the same. Paying your auto loan or mortgage late affects your credit score more than a late payment on your credit card. Thus, pay off the major loans first.
f) You can get rid of all the negatives on the credit reports. This is not always possible. However, you can turn things around by employing good credit building techniques for the newer accounts you have.