The bottom line is that if you want the most natural acoustic sustain, don't play a guitar designed along the lines of a Fender Stratocaster. Get a guitar designed along the lines of a Gibson Les Paul. This is why many guitarists own one of each; they are suited to producing different kinds of tones.
In particular, a guitar with a fixed bridge (hardtail) will have more sustain than a guitar with a tremolo bridge. Tremolos are great for vibrato and dive-bombs and other expressive effects, but this all comes at the expense of long sustain. You can't have both. So when you want more sustain, get a second guitar that does not have a tremolo bridge.
The easiest way to examine differences that contribute to sustain are to look at the differences between classic Fender construction and classic Gibson Les Paul construction.
Fenders and Gibsons
To begin with, a guitar that does not have a tremolo bridge will have much more sustain than a guitar that has a tremolo bridge. But there are other aspects, too.
Every component of a guitar contributes to its acoustic sound. All things being equal, however, most guitar builders have associated more sustain with more mass and density, in the body woods, in the weight of the bridge, the type of metal used, and how it is anchored to the body, and even in the weight of the headstock and the metals used in the tuners and fitments.
Fenders have their own very appealing sound, but many aspects of the Fender design were created not for acoustic properties but to design a guitar that was the easiest to mass-produce. Fenders involve the simplest wood carving procedures. The body design, the way the neck is connected to the body, and the design of the headstock (read below) on a Fender Stratocaster are all designed to require as little wood-carving skill as possible, and to make it easy to interchange parts after the guitar is built. Second, there is the floating tremolo bridge with springs, which reduces sustain. Also, Fenders, classically, have bodies made of inexpensive alder or ash, which have appealing acoustic properties but are not known for producing great sustain.
However, Fenders certainly have their own sound that is very popular--sustain isn't everything.
The classic Gibson Les Paul design was created to provide the most sustain. Its body is carved from a slab of solid Honduran mahogany (rare and expensive, and now endangered) with a slab of hard maple glued to the top. These woods are dense and selected because of their mass and sustaining qualities. The guitar is carved with an arched top to provide the most thickness, mass and density directly under the bridge, and this is said to increase sustain. The mahogany neck is carved using techniques that require a lot more woodworking and skill than the easier-to-mass-produce Fender necks. Significantly, the Gibson neck has a more complicated carved angled headstock, and the neck heel is glued into the body, rather than bolted on. Finally, the classic Les Paul design uses a fixed bridge system (or "hardtail"), not a tremolo bridge.
How the neck is attached to the body
A neck can be:
- bolted to the body ("bolt-on")
- glued in ("set neck")
- the neck runs the entire length of the body ("neck-thru")
The conventional wisdom is that the bolt-on design has less sustain than the set-neck construction. However, various guitar manufacturers dispute this and say that a properly constructed bolt-on neck construction can have just as much sustain as a set-neck design. Everyone agrees that the most sustain comes from the neck-thru design, but this is not common in guitars because it is a much more expensive method of construction.
Set-neck. Gibson's latest design, showing how the neck block, or tenon, fits into the body. The neck is glued to the body.
Neck-thru design in a bass guitar. Notice that the neck is constructed of pieces of wood that run the entire length of the guitar, headstock, neck and body. The "body" is merely two pieces of wood glued to either side of the neck block.
Another factor you have not mentioned can be the angle of the headstock. Fender-style guitars have a headstock whose surface is parallel to the fingerboard. Gibson-style guitars have a headstock that tilts back several degrees, and this is generally regarded as to contributing more sustain. Luthiers talk about the "break angle" of the string over the nut and also over the bridge saddle as contributing to sustain.
The Gibson-style headstock tilts backwards and is regarded as contributing to greater sustain. [Carving a neck in this fashion out of a single block of expensive and rare mahogany requires a lot of labor, and most of the wood is carved away and lost. Today many copies of the Gibson design, including their own Epiphone Les Pauls, utilize two pieces of wood glued together at an angle in a scarf joint to reduce the amount of woodcarving and the amount of wood required, and to improve durability.]
Fender-style headstocks are flat and the string break angle is quite shallow. This is thought to create less sustain. [However, it requires much less wood, and simpler woodworking procedures, to carve a neck in this design compared to the Gibson design. Furthermore Fender necks are made of hard maple, which is not increasingly endangered like mahogany is.]