At a trunk show at JuJu Knits, I was chatting with someone who had frogged parts of a project a couple of times and noticed that the yarn was starting to look pretty worn, so we talked about some things she could do to "freshen" up the yarn. I have two big suggestions, the first if it's a smaller section of yarn that needs freshening up and the second if it's the full skein of yarn.
Smaller Section - Steam
If you have a small section of yarn that you want to refresh, the easiest way to do that is to hit it with a bit of steam. You can use a tea kettle or other kitchen method of creating steam or you can use a fabric steamer.
(Note: Steam is hot! Please be careful when using steam and don't put your hands/fingers directly in the path, especially when using a fabric steamer. I made that mistake for just a split second a while back and it was very painful!)
Pass your yarn through the steam (or pass the steamer over the yarn) until you see it straighten/poof up. You might want to put a little tension on the yarn (pull on it to keep it taut) while you are steaming it. The process shouldn't take very long and it's fun to see the yarn straighten back up as you watch. (If the yarn feels damp afterwards, let it dry before using it.)
(Note: This method works best on natural yarns such as wool or cotton; acrylic is not going to give you quite the same effect. In fact, if you overheat acrylic, you can "kill" it, which means you've melted the plastic and you can't "un-kill" acrylic so be careful. (Note note: "Killing" acrylic can be an effective final blocking for a project that you want to be super drapey such as a shawl.))
Full Skein - Soak
If you have a full skein or almost full skein of yarn that you've frogged and want to freshen up before reusing, the best way to do that is to get the yarn wet and then let it dry.
To soak the yarn, you'll need it to be in a form where all strands of the yarn will be able to take up water. The best form is the hank or big loop of yarn (like most indie dyed yarn comes). So you'll want to get your yarn back into that hank format.
You can use a special tool called a niddy noddy for this purpose. Here's what one of mine looks like:
So it's basically three pieces that form something that looks like an I but one of the short pieces of the I is twisted so that the short sides are perpendicular rather than parallel. I don't have a video myself of how to use a niddy noddy, but I found this one that does a good job showing how to use one. (You'll also see that hers is made of PVC pipe, which can make this a very inexpensive tool if you want to make one yourself.)
However you create the loop of yarn, the most important thing is to place 3-4 ties around the loop so that the yarn doesn't get tangled. Please don't forget this step!
Once you've created your loop of yarn and tied it off in several places, place the yarn is a bucket of water to soak. For a wool yarn, 30 minutes is usually good; if it has silk in it, you might want to soak it a little longer. Remove your yarn from the water and squeeze out as much water as possible. (Just gently squeeze, don't twist and wring it out.)
Put your hands inside the loop on opposite ends (basically, the loop will be around your wrists) and snap your hands apart several times. This "snapping" of the loop helps to give the yarn a little extra straightening. Then hang the loop of yarn until it is dry. (If you didn't squeeze it out really well, it might drip, so keep that in mind as far as where you put it to hang dry.)
Once the yarn is dry, wind it back into a ball or cake and it's ready to use again!