Did you get one of those notices by the IRS that says you owe money for 2005, 06, and 07 but you never even filed a return in the first place?  You’d be surprised, you’re definitely not alone.  Lots of those notices have gone out lately.  If you’ve got several years of back taxes that need to be filed, I recommend hiring a professional to do it for you.  (Okay, namely I think you should hire me, but then again this is my website.)
But seriously, there’s a reason you didn’t file your taxes in the first place; maybe they were too complicated, maybe you were going through a divorce or suffering from a death in the family, or maybe you were just being lazy.  Whatever the reason, the problem has gotten to the point where the IRS is threatening you– so you need to get yourself a buffer zone.  Someone to put a little distance between you and the IRS, it keeps it a little less personal.  Plus a professional will know all those funky little tax law changes:  2007 was the telephone tax credit, 2008 had that $300 recovery rebate credit you missed because you didn’t file, and stuff like that.  You don’t want to lose out on those things.
If you hire a tax professional that’s worth her salt, the first thing she’s going to do is to contact the IRS and get all of the information they have on you.  That will include your wage and income transcripts, your account transcripts, and any return transcripts they may have.  Even though you didn’t file tax returns, the IRS filed one for you, that’s how they came up with what they’re assessing you for.  It’s a waste of time trying to negotiate with the IRS if you don’t know what information they’re using.  Remember, when the IRS files for you, it’s always the worst possible tax status and you get no deductions.
The next step is to prepare all of your income tax returns.  Not just for 2005, 06, and 07—in order to be in compliance (that’s the term the IRS uses for someone who’s in good graces with the IRS) you must have all of your tax returns filed and up to date.  You can’t set up a payment agreement to get yourself out of an IRS levy if you haven’t filed all of your returns.
If you’ve been a good doobie and responded to the first IRS notice immediately, they’ll give you 30 days to file and then you can usually get another 30 day extension before you have to deal with any consequences.  If you’ve blown off the IRS a couple of times already, they will not be so willing to wait for you.  The problem is that you might not know that you’ve blown them off, especially if you’ve moved and they have the wrong address for you.   Don’t assume you’ve got 30 or 60 days unless the IRS tells you they’re giving you that much time.
Each tax return must be mailed in a separate envelope.  People mess that up all the time.  Older returns go to one address, current returns go to another.  And the addresses vary depending upon where you live.  (Another reason it’s a good idea to get professional help.)  Even if you’re sending two or three returns to the same address, you still need to put them in separate envelopes.  (Think of a little kid going through a box of cereal looking for the prize.  Once the prize is found, he sort of forgets about the cereal.  It’s the same with tax returns and envelopes.  Once an IRS agent opens the envelope and finds a tax return—everything else is forgotten, that other return does not exist, only the first one he finds is real.)
Once you figure out what your real tax liability is (remember there will be penalties for late fling, late payment, plus interest), then you can negotiate a payment agreement or perhaps an offer-in-compromise if you qualify.  It all depends upon how much you owe and what you’re able to pay.  A simple payment agreement can be negotiated in about 10-15 minutes, while an offer-in-compromise can take 6 months or even longer.
On the “fun” scale, filing back taxes is right up there with root canals and colonoscopies.  Nobody wants to do it, but you reach a certain point and you just have to.  And, not unlike a colonoscopy or root canal, you want someone you trust doing the work.   If you’re in the “back tax” situation, the sooner you just get it done, the better off you are.  On the bright side, you’ll feel better when it’s all over.