Go to creditboards.com and spend time searching around and picking up tips. There are a lot of things you can do to help your score. Obviously all of your open accounts, make sure you pay on time and try and keep utilization at around 30% - having all of your revolving credit used up will drop your score.
Now, for those accounts that are open if you have any negatives on there try and keep them in good standing for 6months to a year and then you can try and write a goodwill letter (you'll find lots of examples on creditboards) to someone high up in that company and explain your situation and how you've been current and in good standing and would like to see if there is any way that they'd remove those negatives (anything like lates, overlimits, etc)
If you have old accounts that are in a negative standing, or are charged off you can try a couple of things. You can contact the company and request a "pay for delete" where you will pay whatever amount was charged off in full (or if it's a high amount they'll often accept a lower amount) if they remove the listing. Same works with collection agencies who may have collection accounts listed - they'll pay pennies on the dollar for charged off accounts so you can really bargain with them there. Even if you only pay 30% of what the bill is, they're making a hefty profit. But get EVERYTHING in writing before you send any money to anyone. It's also best to use money orders. And make sure that part of the deal is that the amount you agree to is accepted as full payment and they are not allowed to sell any remaining portion to another collector.
If the accounts are charged off and past statute of limitations (check your state - some are only 3 years, some up to 6 years) you can try and dispute the account with the credit bureau (not mine, no knowledge of account, wrong information) A lot of times companies will purge old accounts from their system so when the agencies call to verify, there's no record of your account and they can't verify so the bureau will have to delete the tradeline. If you try this before statute of limitations you may be sending up a red flag for collection agencies.
I wouldn't use any company that will "fix" your credit for you. Some are good, but you can do the same work yourself and REALLY know what amount of your money is going to who. Just spend some time searching through those boards and learning what's worked for other people. There are a lot of people there that really know what they're doing.
Good luck!
Carrie