Without being surrounded by efficient insulation, your shelter is worthless. Both in winter and summer.

Since we built with the otherwise brilliant pier foundations, we didn't have the support to incorporate literally tons of thermal mass in our buildings. These structures are lightweight. No poured concrete floors or block walls, no huge masonry fireplaces. And without the mass such items add to a building, there's little incentive for either hot or cold air to stay put when you need it to, making a true passive solar design impossible.

This embarrasing fact makes good insulation even more important. Or so I think.

WelI, I decided to super-insulate. We used to live in a small strawbale cabin that we built in the California desert. It had, among other neat features, 24" thick walls and a R-40 roof. Cold, heat and even a couple of major tremors mattered little when sitting snug in there. It showed us the potential of a home with adequate insulation.
For various reasons I didn't want to redo the straw thing here, so we were looking for a way of doing thick, soundproof walls of about R-35, with regular framing. The solution is 2X4 double walls. Read how to build them here.
With this concept you use regular 2x4's from small diameter trees, a little environmental feel-good incentive. Also you avoid the thermal break of having a thru the wall stud every 16". Walls are light, making them easy to raise alone if need be. Running electrical wires in the space between the two walls couldn't be easier. Window sills are 8" deep. Cost is low. So doing this was a no-brainer.
In the roof I experimented with a similar thing. The 2x8 rafters has room for a decent amount of insulation but with the everpresent thermal breaks. Therefore, hanging underneath, offset in relation the 2x8's are 2x4's, whose sole purpose is to hold the extra batts. The constuction details of this is touched upon here.
The plans specified minimum 2x6 joists for the floor platform, but this would only give us R-19, skimpy and out of context. 2x10's with their R-30 potential sounded better. This is the first insulation to go in, way early in the process, before building walls and putting on the roof. I had some concerns about keeping things dry down there for who knows how long, so this is what I did: After dropping in the batts and nailing down the 3/4" plyvwood I smeared a bunch of caulk in the seams, and coated the whole deck with copious amounts of discounted latex paint.
Incidentally it did rain hard before the roof got on, but I didn't have to worry about the fluffy insulation turning into a lifetimes worth of moldy mush. So this is a good idea to pass on.
Do it your self'ers with stick frame structures are limited in their insulation choices. Although blown in polyurethane foam may be very efficient, getting it in the walls takes more infrastructure than most of us have. Essentially we're left with the batt option.
I wanted to be meticulous with the installation, but knew that would not be possible with fiberglass. You just don't hang around that stuff for long, fussing endlessly with filling little cavities. It is a very unestethic material, besides the obvious health risks, yet insulation is so important.
The alternative is cotton batts, made of processed scraps from the denim industry. We used this product everywhere except in the floor platform. It is soft and supple and safe to work with. Cotton is a little denser than fiberglass, so you have more control when placing the batts. It recover well from compression and holds together nicely. Scraps are infinitely useful for everything from dog beds and pillows to insulating pipes. Off course it is way more money than fiberglass, and next to impossible to find. Luckily, we live 4 hours from a progressive metropolis where such things are hugely in vogue. Regarding the cost, just apply the small cabin mantra: The quantities needed are so small that stepping up in quality can be realistic. Even on a budget.

Besides filling all cavities with fluff, there are a few other things to consider.
Windows bring in light and we all like having plenty of light. But they are also very poor insulators. It is worth while to invest in nice tight fitting honeycomb shutters. This 3/4" double cell model cost an average of $80.00 per window. They come in either blackout or light filtering versions.

We have hot and dry summers. Keeping the cabin cool when it is 101 degrees F is challenging for the same reasons stated above: Lack of thermal mass within the envelope. The most successful step we took against overheating was choosing a light colored metal roof. It doesn't fullfill the stereotypical mountain home look, with their generic green lids, but it really works in reflecting the hot sun. Form follows function. Always.
Also somewhere in the roof is a layer of foil faced reflective bubblewrap, which may or may not be effective.