Worn-out cycling shorts that have gone a bit too transparent but are otherwise perfectly functional, under something respectable looking. Either looseish civvy trousers (though I find saddles destroy trousers suboptimally quickly) or unpadded legging type things (Bikesters, Altura Cruisers, that sort of thing), which are quicker drying and a bit more durable, but don't really scream 'lycra lout'.