This would be difficult, but not necessarily impossible. Without more information on what's dragging you down, it's hard to say. Some suggestion.
Reduce your balance on all revolving account to 5% or less.
If you have any collections open, send PFD letters. Do not, I repeat, do not, call any of the collection agencies or original creditors and pay them with a promise of delete. Get it in writing.
Any lates ou there, send GW letters. If they don't work, try again.
Avoid any inquiries on your accounts.
Do not open any additional lines of credit.
Now here's the thing. If you have collections and lates, you're essentially at the mercy of the people who placed those on your account. And some of them may simply be unwilling to play ball.
Your timing, three months is also kind of interesting. If I were to speculate, I would say you're trying to get your score high enough to get a home loan in time for the 8K tax break. If this is the case, you may run into some additional problems. Your lender is not going to just look at your score, but they're also going to look at your history. If you've been sent to collections, or had late payments in the past 6 months, you're going to have problems regardless of your score.
Sounds like you're well on your way then. If you do some searches on your various creditors, you can probably find addresses that people have had success with sending GW ;etters to.
Personally, I've had success with Chase Auto Finance, but no such luck with BofA.
One other thing to note, which I mentioned in a previous post.... I had 4 lates with Chase Auto Finance. After I GWd them, they sent me a letter back indicating they'd remove one of the four. Since it was from an individual within the company, I went ahead and sent him a thank you letter. A few weeks later, I find out that they deleted three of my four lates. Was this because of the thank you note? I will never know. Couldn't have hurt though.