We kinda did this. We had to de-clutter to get our house ready to sell (less stuff makes the home look bigger to buyers!). DH (at the time he was my fiance, though) already had a storage unit for his bachelor stuff, so we just moved everything there until we moved to our bigger home. But the same princeple works if you are getting rid of junk.
Start in one room, make two piles, "Donate/Garage Sale" and "Throw Away". Anything you don't want or won't use that is still in good shape, put in the "Donate/Garage Sale" pile, the rest goes bye-bye in the trash. Once out of the way, go back and start to reorganize what's left (sort, file, put away etc.). When sorting through what to keep and what to get rid of, be VERY realistic about if you REALLY need it and/or are you REALLY going to use it for something. Especially with clothes. If you haven't worn it in a year, you won't ever wear it again, so send it to Goodwill.
Once done in one room, take on the next. Don't do more than one room at a time if you can help it.
DON'T spend money on 'organization' systems until you clean out the crap first. Once you see what you have left, you may not need it, or if so, you'll know exactly what you'll need and not buy stuff you can't use.
As for the Donate/Garage sale pile, obviously it's up to you to either donate it or have a garage sale. If you live in a cold climate, it would be easier to donate it then try to have sale, but try to do it by the end of the year so you can take the deduction on your 2005 taxes. If you donate in January you'll have to wait a year and take it off of 2006. Of course if you don't itemize then it's doesn't matter!