1. Set up an account at Credit Karma.
2. Review your Credit Card Utilization. If it's under 30%, you should be doing ok. If not, pay off those balances.
3. Look for derogatory marks. If you see any that are older than 7 years, file a dispute to have them removed (you can do this through CK directly).
Generally, I found that, if an account is closed and older than 5 years, if you file a dispute, the creditor won't care and will either ignore the request (resolving in your favor) or mark it as resolved in your favor.
If you have outstanding balances open in collections, call the collection agency. Here's how that went for me... "Hi, I'm interested in discussing an open collection on my credit. I see you are the owner of the debt." After they pull up the information, ask them "How much are you willing to accept to purge this from my credit?" (They are going to tell you they can't do that. But, they most certainly can!)
If you receive an offer for 20% less (with removal from your credit), take it. Use a credit card (responsibly!) if you don't have the funds immediately available.
If not, make sure they have your phone number on file, hang up and wait. Come the end of the month, I guarantee you they will call with an offer. :)
Tip: Use a Google Voice number, so you can field their phone calls, if they go rogue. When you're negotiating, make sure you write down names, case numbers, account numbers, etc.
4. If you have old accounts on your credit report, don't close them! Closing your accounts hurts your score. Instead, keep them open. If you have a credit card that you never use, set up Netflix to bill to that card, then set up recurring bill pay to pay off the balance every month. This will keep the creditor from closing the account due to inactivity. That credit history will keep your "Age of Credit History" up.
Lastly, if Credit Card Utilization is constantly hurting your score. Contact your existing credit card companies and request a credit line increase (don't open a new card!). A credit line increase will not hurt your Age of Credit History, but a new card will.