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.
I've posted this picture before. It folds up so small that it's just not worth it to put it in a saddle bag or strap it under your saddle with all sorts of funky straps and paper.
It goes right into your center jersey pocket and still plenty of room left for other things.
I dont put glue on it, to ensure I can fold it up as small as possible. If you ride carefully the tire pressure alone is enough to hold it onto the rim and get you home safely.