Planar Travel
Sigil Prep, of course, is in the Center of the Multiverse, but the Multiverse is pretty big. And not all of it is connected properly. So how do people get from all over the known planes, to the school which for five years they'll call home? There are a number of means.
PORTALS
Portals are common in the City of Sigil, and they go everywhere. Many have been collected or created in the Hall of Portals in the Administration Building, keyed to your student ID. It's as easy as that to get home, or go on Spring Break in Xen'drik. Most school portals in the large cities on the core planes (Eberron, Faerun, Greyhawk, Krynn), are attended on the other end as well by school employees. On the other hand, a number of school portals dump out in swamps, sewers, and slaughterhouses, so make sure you know where you're going.
There's also Jov's House o' Portals in town, for portals to smaller planes, or odd locations on larger planes. Jov charges a pretty penny, though, so don't use it to go to Cormyr or Sharn.
PLANEWALKING
Walking the Planes. Great fun, but is it practical? Each Material Plane has its own cosmology. Spells such as Plane Shift, or even Gate, only work within a given cosmology, so you can shift from, say, Greyhawk to Sigil, but not from Eberron there. Some planes co-exist in multiple cosmologies, which makes things somewhat easier. Shift from Faerun to the Abyss, then from the Abyss to Sigil. Bingo. It gets harder with places like Eberron or Rokugan, which don't share any cosmological points with other planes, so portals become your best option.
Transitive planes (Astral, Ethereal, etc) go everywhere, but you have to travel to get from one plane to another, and the distance is often huge. It can take several years to Astral Travel from Eberron to Mystara. It's a pain. Portals, portals, portals. Don't get cute.
SPELLJAMMING
For several years, Spelljamming had been disproven, and stopped working. However, it has now been reproven, and works again. All Material Planes are encapsulated by shells called "Crystal Spheres". Spelljammers allow people with a boat and a helm to travel through the space between the prime world of their plane, through the Wildspace of their plane's sphere, and pass through the sphere itself into the Phlogeston.
The Flow (Phlogeston) has been recategorized as a transitive plane. It only touched material planes, but with a Spelljamming vessel, it becomes possible to travel from any material to any other material plane, which, barring portals, makes it the best way to get from relatively shut off cosmologies like Eberron or Athas, as opposed to planewalking, despite what the 'Walkers would like you to believe.
Helms
Any ship can become a spelljammer, simply by attaching a Helm to it (though many are created specifically for that purpose). (These rules are greatly modified from the 2e AD&D rules because this isn't 2e, and also because nothing I've seen takes into account new options, such as Incarnum and Invocations. Blame it on the change in theory). A Helm must be manned by a magic-user of some sort to enable the ship to fly. Once the Helm is manned and activated (Spellcraft check DC 15), the ship is capable of flight, and can be guided by the Helmsman. The Helmsman's senses encompass the exterior of the ship, and she can see in all directions around the ship. The Helm grants ships gravity relative to the recognized "bottom" of the ship (or toward the center, if that's the way it's designed), and a breathable atmosphere. The Helmsman can also will a portal to appear in the crystal sphere's wall to allow entry and exit from the Flow.
The Helm is a chair, which the Helmsman sits in. Minor helms can only power crafts size Huge or Smaller. Major helms can power any size ship. There are several varieties of Helm:
Standard Helm: Powered by spells, and various spell-like effects (utterances, mysteries, infusions, spell-like abilities). The Helmsman feeds his spells into the Helm. He may choose which spells to feed it (specific prepared spells, or a spell-per-day spot), and it can power the ship's movement for one hour per spell level. The ship moves, within Wildspace or atmospheres, at a speed relative to the level of the caster.
Psionic Helm: Like Standard helms, but powered by the manifester's power points. Each power point powers the ship for one half hour.
Invocation Helm: Like a Standard helm, but it powers the ship by drawing invocations. Due to the means of drawing power, however, the Warlock must select an invocation to feed the Helm, and the Helm draws from it. The warlock cannot use that invocation at all until he rests, even if he gives up the Helm. Least invocations: 1/2 hour. Lesser invocations: 2 hours. Greater Invocations: 4 hours. Dark Invocations: 6 hours.
Incarnum Helm: The incarnum user must attach the helm to his crown chakra. It draws from the Helmsman's essentia, allowing 1 hour of power per essentia point fed to it.
Ki Helm: Kung Fu! Martial adepts can use their ki to power to power a Helm, spending a maneuver in the same manner that the Standard Helm absorbs spells.
Leech Helm: Any corporeal being can power a Leech Helm, except undead or constructs. It works like a Psionic Helm, except it draws hit points instead of power points. It does not require a Spellcraft check to activate.
Undead Leech Helm: As the Leech Helm, but it is powered by Undead.
Artifice Furnace: An unmanned Helm, the Artifice Furnace must be fed magic items, which it consumes to create power. Each magic item powers the ship one hour per caster level. Minor artifacts will power the ship for double the time. Major artifacts will power the ship a full week per caster level, but even these will eventually be consumed.
(There is not presently a helm type that utilizes Vestiges)
Anything that would disrupt a caster's concentration may also disrupt a Helmsman's concentration. This includes damage to the ship itself, which the Helmsman can psychically "feel" (even though it does not damage her). If the Helmsman loses concentration, she must make another Spellcraft check to regain it. A suddenly unhelmed ship will slow to a stop in Wildspace, or drop within a world's atmosphere.
Most Helms are able to hold enough risidual power to allow a ship carefully lowered in place, to hover above the ground.
Phlogeston
Once out in the Flow, there are natural currents and "winds", which allow the ship to be sailed, almost as if it were on the seas. The Helmsman can rest, because the ship can be guided by wheel and sails. The Flow is flammable, so fire spells and effects that extend past the ship's atmosphere (10' from the ship's edges) are always Empowered, Maximized, and Widened.
Ships in the Flow travel at the speed of Plot.