This is a partial answer, to supplement the answer by @wimi.
Your options for the duchar example are
with the latter preferred. You wouldn't say "A ducharte" because in this construction the verb is expressed for a general person, not for a particular person. (Note this is consistent with what wimi said about Neruda's work.)
I would generally prefer the latter option, "A ducharse," because the shorter one could sound abrupt and a bit like a drill sergeant barking an order. However, sometimes in family life one needs to minimize the number of syllables and so one might say "A duchar."
My personal opinion about the negative is that since this construction is a call to immediate action, in general the negative doesn't work. That's what makes "A no preocuparse" charming. It is a breaking of a grammatical rule, and the irony helps the worry wart laugh. As a parent of a child with OCD, I've seen that the laughter distraction is one of the best ways of coping with worrying.
As a more common example, consider the command to not pay taxes. There are no slogans or posters saying "A no pagar impuestos." Instead, the imperative form "No pagar impuestos" is used.