I'm not sure why I've never seen this mentioned in this forum, but I learned of this many years ago. I didn't do it, but this passt year, when my brother was rebuilding his credit, I told him about this quick fix. He did it and it bumped his score about 50 points. It's easy, quick and I've never heard of anyone not being approved.
Go to the bank or CU that you bank with. Open a 6 month, $500 (or other amount) CD. Then, open a new loan with them for the amount of the CD, with a 6 month term, pledging the CD as collateral for the loan. Since they hold the CD, I've never heard of a bank refusing to do this. They have no risk. Just be sure that the loan is for the amount of, or less than, the CD and that time limits match. (6 month CD=6 month loan, etc.)
Now you have a new installment loan from a solid banking institution on your credit report. Now, this is important: Do NOT pay it off right away. On the fifth month, it's okay and it will show you paid a little early. That's not a bad thing. But if you run right in during the second month and pay it off, you haven't done yourself a favor because it doesn't show that you can make payments on time over a period of time.
After you've paid off the loan, do what you want with your CD which, by the way, has been earning interest for you during this time, albeit a small amount these days. Roll it over, cash it in, or pledge it against another loan and start the whole process again.