I have raced and trained exclusively on tubular tires since the early 80's. In all of the kilometres I have ridden (training and racing) I have only ever had four punctures and on two events, with consecutive punctures of two tires within a 200m section of road
. In both cases, the punctures occurred while training and in the rain or sleet. Since I only had one spare in each occasion, I was SOL. When it happened last season, I just had my wife drive out to get me, as I was still 80km out.
My point is, one of the main reasons that you ride tubulars is that they don't get flats. Having said that, for training rides I always take an old, well-worn tubular (you will get lots of those, as they invariably get retired due to the tread being worn off because they never flat
), fold it by rolling it around the valve stem in 10-12cm increments, wrap it with a paper bag (to keep sun and junk off it) and secure it under your saddle with an old toe clip strap (if you are old enough to remeber those).
Oh, don't forget a pump. I like the alloy Toppeak ones, as they fit neatly between the rear quickrelease and the junction of the seattube and seatstays. Don't be swayed into buying a tiny 'mini-pump'. If you ever do get a flat in inclement weather, you will hate your life trying to inflate to pressure with one of those things.