Hi,
I just had a garage sale with the fruits of my decluttering efforts (against FLY lady's advice). All of your worries are well founded, it is A LOT of work and it definitely keeps the crap in your house that much longer. My fiancee and I started having a lot of stress and arguments over it.
BUT, we made $500 on our tag sale, which we are putting toward our wedding. I don't have many opportunities to bring in extra cash like that, so it was very nice. That is definitely more than I would have been able to deduct by donating because many of the items would not have been accepted by a charity (an opened bottle of perfume, for example). We donated what was leftover so as not to have to bring it back into the house.
We (I) decluttered nonstop for two months, and it really was getting to be a big burden. Crash and burn is hard to avoid. Be aware that you will feel more stress and impatience during the decluttering process knowing that you are trying to have a sale in 1-2 months. If you are familiar with FLY lady then you know she teaches you to do it 15 mins at a time, baby steps. This will not be baby steps, it will not be easy going.
But if you can handle life getting VERY hectic for 2 months, the rewards (monetary and a quickly decluttered home) are very nice. Also, for me, I may have been more ruthless with my decluttering efforts because I wanted to have a lot of stuff for my sale, so that was a good thing.
Just make sure you know what you are getting into. You may want to check out the forums and tips at
www.yardsalequeen.com
If you do have the sale, then like many of the posters have said, after the one big sale, just donate things. Freecycle is great.
Hope that helps - good luck and stay strong!