Although I personally have never had my credit score increase in 30 days some people have reported that they were able to increase their credit score by 100 points in 30 days.
The way that the people have increased their credit score by 100 points in 30 days took some work but here's how they did it.
They got a copy of their credit report, looked over anything that was in collections and contacted those collection agencies to see if they would remove any collections accounts from their credit report if you paid those collections accounts off.
If they say yes they'll remove the collections reports then you should pay them, if they won't remove the collections accounts from your credit report then don't pay them and move onto and pay only the ones that will remove your collections from your credit report once you pay them.
Next you should dispute any negative remarks on your credit report to get them removed.
A secured credit card can also help you improve your credit score if you don't have a credit card and cannot get another one due to bad credit.
Check out Capital One Secured credit cards where you put up a refundable deposit and then Capital One gives you a credit limit and you use it in a responsible manner and pay the credit card bill on time and keep your balance paid off.
The secured credit card will help you boost your credit rating so you'll be able to get approved for other loans and and non secured credit cards.