It's taken me a while to learn that when I've made a mistake and have to unpick it's generally a good idea to do it straight away. The chances of me going back to the "Almost finished except for that one disastrous mistake"-pile after months have gone by are pretty slim.
After once losing my only seam ripper just when I needed it, I went and got three so that I can be sure there's always one handy. It's great to know that it's never a disaster if you sew a sleeve onto the hem when hemming (yes I've done that!) or attach the bias binding on the wrong side of the garment (done that too, then unpicked and sewn it on the wrong side again). This always seems to happen when you're just about finished and want to go out wearing your creation!
The quickest way to unpick a seam is to cut about every fourth or fifth stitch. Despite the name there should be no ripping involved. Just a gentle push should be enough. If you feel like you have to force it, switch to a new, sharper seam ripper. Any pulling might damage delicate fabrics.
Turn to the right side and pull the thread gently. It should come off easily and with it the thread bits from the other side.
Now you just need to pick the thread bits. A lint roller works great for this. Press with steam to eliminate the stitch marks.