I have a very nice sleeping bag which has been kept stuffed as compressed as possible in a backpack for a couple of months. After reading about the negative effects of such actions, I was wondering if there was anything I could do to return it to its original poofy-ness.
Original Poofiness (or OP as they call it in the 'wood) will always be illusive. No matter how you care for your bag, it is on a steady decline toward compression from the moment you buy it.
To attempt to re-gain some loft, you need to separate the fill so the fibers or down (or whatever) that are inside can trap more air between them.
- Shake the bag vigorously from all directions
- Fluff the bag like a pillow -- slap, punch and clap it
- Feel for clumps of down in the bag and gently pull them apart with your hands
- Place the bag in a large commercial tumble dryer on air (no heat) with a couple of tennis balls tossed in
Of course, as you mention, never store a bag compressed in the first place.
The impact of compression on loft can come from how long a bag is left compressed, or from the number of times it has been compressed. As the main risk with compression is the breaking of fibers that would otherwise contribute to increased loft, many repeated cycles are more likely to cause trouble. That said, synthetics and down are different.
Synthetic bags: As the fibers break down over time, synthetic bags will lose loft and eventually need to be replaced. You can try fluffing up the bag, and a bag that is relatively new but just compressed for a number of months is likely return to close to its original loft.
Similar to down bags, synthetic bags will also lose some loft as they get dirty with use, so eventually you will want to wash the bag. This will help restore some of the loft.
Down bags: Refer to Dakatine's answer to Will a down sleeping bag recover 100% after being squeezed if it's not for a long time? Down is much more durable than people often give it credit, and is quite compressible. The stiffer feathers may break from compression, but the down itself will be fine. (Thus, a higher fill power down will compress better and be more resilient.)
For down, loss of loft over time is most likely due to the bag getting dirty (e.g. from skin oils) and the down clumping up. You can try simple fluffing methods (tumble dry, low or no heat, tennis balls), but eventually you will want to have your bag washed. This is a topic for another question and is a rather delicate job, as you can easily tear the fabric if you're not careful.
If your down bag is clean and dry, and the only issue is that it was stored compressed, your main problem is that it will likely take more time to loft. The dryer method should assist with this, or you can just shake it out, spread it out, and let it sit for a few days.
Something to be careful of is over-compressing wet down, or leaving damp or dirty down compressed for a long period of time.