If you have not heard of interior storm windows, maybe you have seen people in cold climates cover their windows with plastic to stop the draft coming in? This is like that, but better. I have built frames that are then wrapped on both sides with shrink wrap plastic so you end up with a double glazed plastic window insert. The frames are made 1/2" smaller than the window openings. This enables them to be edged with 1/2 by 1/2 foam weather stripping that makes them a tight press fit into the window. It also seals the window area, and makes a big difference in air infiltration. With less air flowing into the house, there is less hot air being displaced, so the house stays warmer, and the heat runs less. With a cost of between $10-20 per window, this may seem like a large outlay of money, but my dad, who has also done this, said for the 60ish windows in his house, it paid itself back in under a year. While I have not kept detailed records, and also have not done a winter in this house without them, I cannot speak to how quickly they will pay themselves back. On the other hand, I can feel a marked difference in how warm the house feels, and how long it takes for the heat to kick on.
On a tip from my dad, I bought pre-primed, finger-jointed boards, as my windows are white. This means that I did not have to paint them to match, which cut down on the time it takes to cut, assemble, cover, and install them. You do have to be careful when assembling them to make sure the primed edge is facing in, as I ripped 1x4's in half, so one edge is bare wood. The added complication for me was that I used half lap joints to assemble them (stronger than butt joints), so I had to consider which side the dado's were on. Not all of my interior storms ended up with white borders on the inside, but it is trivial.
Because these windows are covered in shrink wrap material, if a storm window has a dimension of more then about 50", a cross piece is necessary. If the crosspiece is not used, the shrink wrap will pull the center of the frame inward, making it no longer line up with the window frame (read: it leaks air, otherwise).
If you do a good job measuring where the crosspiece gets installed in relation to the window, it essentially disappears where the two window halves meet. This makes the interior storms almost invisible. 
On the other hand, if you are installing one where no one is likely to see it, who cares what it looks like. This is covering the hatch to the attic crawl space. I attempted to weather strip the hatch and insulate it, but both were poorly executed due to space and time constraints. The hatch is the weak point in the attic insulation at this point anyway, since I blew in about 25 bags of cellulose insulation this fall. When the house was inspected we were told there was only 0-10" of insulation up there. In climate zone 6, code for attic insulation is r-49+. With a maximum of r-32 up there, we were deficient in insulation, especially when you consider that over the dormer-ed sections had none. By cutting access holes and blowing in 25 bags worth of insulation, we are now above r-49 pretty much everywhere. The only places the are deficient are the attic hatch and the cathedral sections. Those are posts for the future though!
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