i'm sure that we've all played a round of Dustbowl on a disorganized public server, one in which half the red team rolls Engineer. then we have a whole mess of level one sentries at the front with all the engineers bickering over the metal, and they soon get wiped out, the first point is capped, and we scramble to defend second. then the noobs decide that playing engineer sucks, so then they all go as soldier or pyro, and we wind up with one engineer on a 24-player map (two if we're lucky), and once again, the point is capped tout suite.
of course, the obvious answer is - have a handful of players go as engineer, and stick with it the whole round (and if your sentry gets blown up, just build it again, don't ragequit your class).
but then i came up with another idea - what if you have 2 or 3 guys go as core engineers (the guys who will be in charge of sentries and other buildings the entire round) but have 1 or 2 other players act as "support engineers" - engineers who don't set up any buildings of their own, but help the other engineers build stuff, maybe even venturing out into the battlefield once the gates open to pick up weapons while the core engineers stay safe and protect their equipment. i wouldn't recommend this for the entire round, just until the core engies get their teleports and dispensers up, and their sentries at least to level 2, which can happen very quickly with a couple extra engies working on them. then the support engineers would just change classes to skirmisher or support characters such as snipers, spies, pyros or scouts, which won't really be that useful during the initial uber-rush where it takes medics with full uber combined with heavies, soldiers and demos to hold back/ break up the ubers.
i wouldn't reccommend this on ctf maps, or in rounds where you have less than, oh say, 20 players, but otherwise, it might work...