The second method is to suck hard briefly, drawing the water back into your mouth. Some people find this latter a bit repulsive, but it is only water and you produced it in the first place so it won't do you any harm.

Not only is it "only water" it is actually rather purer than the water you get out of a tap, unless you've used one of the surfactants described in the various posts above. Antarctic expeditioners quite often are seen with it frozen into their beards and moustaches and I can assure you that when you suck on it, it melts into your mouth with no taste whatever.

Hold the whistle with the mouthpiece tucked into your armpit - it keeps it warm.

When daughter #2 is presenting for eisteddfods or exams (she's on the Northern Tablelands, where temperatures can be cool, and is a demon on the recorder) I usually put the whole recorder up my sleeve, especially the treble, and even the tenor if the ambient temperature seems at all cool; even descant recorders have a higher 'thermal mass' than tin whistles because of their more 'massive' construction and such prewarming minimises condensation.

Another non-toxic wetting agent is the stuff they use for cleaning contact lenses.

As is the 0.05% methyl cellulose used as a wetting and lubricating agent used by wearers of 'hard' contact lenses. And, if I were to use pipe cleaners or anything else poked up the barrel of a whistle, I'd be careful to avoid contacting the fipple with it; they tend to be sensitive little beasties.

Perhaps in response to my earlier request, someone could make a few suggestions about tunes (on harmonica or whistle, either originally purely instrumental or vocal) which are reasonably easy to play but sound reasonably difficult

Not being an actual player of whistles I'll leave specific suggestions to others but, as a general principle for any instrument I'd suggest trying tunes of songs and gaining proficiency on them before trying out dance or session tunes, which tend to be full of little notes played fast. It worked for me on harmonicas where, as an anglo concertina player (who knows his way around dance and session tunes) I limit myself to song tunes.

Cheers, Rowan