I have a cavalry sabre that I'm interested in identifying and restoring. I know a bit about swords, but not a lot, so let me know if I'm leaving out something important. I don't have the ability to post pictures, i'm afraid, but I'll try to describe it as best I can.

The sword belonged to my grandfather; from talking to family members, it seems that it may originally have been owned by his father in the Spanish-American War, but I'm not sure. It was kept in a barn for 40 years; the grip and hilt are discolored and the scabbard is badly rusted and pitted, but the blade is in reasonable shape. It's speckled with rust along the enitre length, but the etching is still quite visible.

The blade is about 30 inches long, measured from the hilt, with the point curved two inches back from the base. The hilt is about five inches long, and made of (I think) brass, with a wooden grip. The grip has deep finger grooves and is clearly right-handed; I'm a leftie, and i noticed that it doesn't fit as well in my left hand. The blade has some nice etching along the first two-thirds of its length, various flowers, cannons, shields, flags, etc. framing, in the center, a bald eagle with a stars-and-stripes shield and a banner reading "E PLURIBUS UNUM". The other side has a similar theme, but replaces the eagle with "U.S." in ornate Gothic script, and near the hilt says "SPRINGFIELD ARMORY SPRINGFIELD MASS. U.S.A." The blade is blunt; I don't know how to tell if it has ever been sharpened.

I'd like to find out for certain what the sword's provenance is, and, if possible, have it restored it to a good-looking condition. Any suggestions for how to go about this?

From your description, I'm going to guess that what you have is a model 1902 officers sword.

Here is a link with some brief info.

US Model 1902 Army Officer's Sword Identification

Restoration and conservation are different paths you could follow. This would probably be a better
candidate for conservation.

Gentle cleaning. The blade I would first rub with an oiled cloth. You could go on to more agressive cleaning with very fine steel wool but I wouldn't. You want to stabilize the rust and go from there. The scabbard might be easier to think of refinishing but most don't, on antiques.
Here is another link with a good few do and don't tips http://members.aol.com/machood/swordcare.html the main page of that site is worth a good look as well.

The 1902 was made in great numbers and (I believe) is still a US military sword (Marines?).
Edited to add that after a bit more looking it turns out to be the current West Point cadet sword, not a Marine NCO item.

There are a lot out there on the market in clean to pristine shape.
Cherish the heirloom but don't get too carried away with trying to make it look new.

Cheers

GC