How can I make my credit score jump 40 points quickly (and how quickly can this happen)?

The last 3 years I have been slowly bringing up and fixing my credit. Now, I may have a great opportunity to buy a house under market. Unfortunately, I am 40 points low on my credit in order to get approved for a reasonable loan. I know about paying off balances on time and not opening new accounts, as I have been doing this already. I've also disputed things on my credit report, so I don't think doing that again will work. I am not interested in paying a "credit fixing" company to do any of this either. So to rephrase all this in simpler terms, I have already fixed my credit, but need another 40 points in this last stretch. What tips/experiences do you have to try to do this in a timely manner? Otherwise, how long will it take for my credit to go up if I continue following the rules of credit?