Some people carry tubulars in their jersey pocket, but you have better use for that pocket, plus the tubular (if it's glued up and presumably a used tire, so somewhat soiled) will tear or soil your jersey. Who has time for that? Also, the tubular should really stay dry.
Most of the bags in the local bike store that are big enough to fit a tubular are pretty short and ugly. You have several alternatives. First, a toe strap (the Binda laminated off-white toe strap is de rigeur, though a double track toe strap can look pretty cool as well) to hold the tire under your saddle does a nice job. There's a cool guy's way to fold the tubular (http://www.velocipedesalon.com/forum/f7 ... -2283.html
) for this application, but you can fold it up any way you want as long as you aren't getting glue all over the bike or the casing of the spare (I know, your glue should be dry, but if you fold it so your gluing surface is pressed against your casing or tread, some glue will transfer). If you're in southern Cali, you can strap it under your saddle without any cover. If you're riding in Central Park these days, you need a little more. Also, a toe strap or whatever you choose to use can cause the tire to chafe, which isn't good.
Next step up is to put the tire in a sock. I always thought that was funky, but some people find it works. It doesn't keep rain off the tire, it's bulky, it doesn't look as sharp as your carbon fiber frame, etc.
Next, get a piece of cordura fabric and wrap the tire neatly in it, then strap it in. This avoids chafing and keeps the tire cleaner.
You can get a Tufo tire bag, actually made for tubular tires and very nicely done (it's actually made by Jandd, who make very nice bags, with the Tufo logo embroidered on it). I believe you can get them still at www.worldclasscycles.com
. It holds almost any tubular plus a couple CO2 cartridges, a tire lever or two to get the tubular off, etc. It actually looks like a tubular tire bag used to look, and since part of why we're riding tubulars is to be cool, don't you want it to be obvious?
I personally think there's a really nice way, especially for weight weenies, to do a spare tire. It's the one tire in the Tufo lineup that I really like, the S-3 Pro. It's only 195 grams, it folds down very tiny because there's no stitching and seam under the base tape, and although it's very thin and supple, it's also got a tough rubber that gets you home. Many people use big bulky cheap tubulars as spares, but it's worth it to get one of these, stretch it well (they can be tight to mount the first time), and roll it up tight. Because the casing is synthetic, you don't have any problems if it's rolled up for a year under your saddle. It's small enough to fit in a moderate sized saddle bag and then I simply fasten the whole saddle bag to the saddle rails with that toe strap, which compresses everything very nicely and makes it very secure. You'll hear all kinds of disgust about Tufo tires on this website, but this one tire is really nice for what it is. It isn't a Veloflex Record by a country mile, but it's a light, very compact tire that makes a great spare. Don't knock it.