Great GREAT question
And then I saw most of the answers (all of the long ones), which may even be Better?
I have just passed the threshold where I am as likely to get three or four compliments or even "don't stop!"s in any given year as I am to get a single impolite "turn it off" or "shaddup" or whatever. (Whistler, not hummer, much. Inhaling and exhaling (which made pianissimo high register easier 'til 2018, when that last new outward-whistle skill found me, for some reason) and sometimes with accents or a strophesworth attacked 'stridulescently' -- by pronouncing bilabial-not-labiodental Japanese v=w for full-scale puckerwhistle+fixed-buzrate-drone.))
My epiphany RE that same Q. was finding myself having unknowingly started to whistle, idly practicing no recognizable exercise to myself, somewhere outside, like I probly did a bit too much when alone at home (the idly part); but here was different... I like to be ultra-polite, tho', to the random unmet world at large, so I made my first**
- POLICY FOR OUTSIDE rule: Public spaces are for rehearsal-type whistlage, not excercise-type or newshit-exploration noizage.
That made sense to me when where someone might come around the corner at any moment, or where I might have just walked past houses with windows right on the sidewalk. With the rule tho', now If I startup whistling unconsciously, it'll at least be recognizably truly music (like any of my fellow hoodmates bangin' the soul-n'-oldskool or whatever), albeit likely an original or classical or a jam with whatever other ambient sound or noise or radio my walking steps are bringing me towards. (Gentlefolk play along with others' beats/sounds improvisingly, sharing the public soundstage. BEEPING CAR HORNS & BACK-UP SIGNAL KLAXOfany must be OUTGUNNED in countertempo riffage, kept at persistently until they s.t.f.u)
SOUND_GOOD only need 2 (TWO!) keypoints to fix our rep A.K.A. WHY 80% OF PEOPLE WHISTLING ANNOYINGLY don't, AFTER BEING LIED TO educationally (#1): "You sound pretty good, Man, fo' sho'; hold those notes longer though... they're gone before we c'n hear it..."
Begin-termediate key the first: Quickly clamp-your-onbend (if any) at "nailed-it" pitch, then hold on-pitch way longer than you think, slowing tempo even, if you're not at least highest-high intermediate, and if you are, well melisma-loud-LOUD, cauuuuuse you must be jamming: Jazz, riffin' with a combo or tape, right? Or Carnatic? Not an unrecrafted horn solo, I truly hope. Begin-termediate key the other-first:for advanced/advancing beginners, in partic, even /more/ important Do your stuff with far less measure-for-measure vibrato (none 93%+ none-ish 5%) on most notes/phrases, and then use it for real for accents and long cadences or whatever, in ways that feel meaningful for whatever vibe-nuance you like to express and can "feel" in doing it with.
Whistle in and out, alternatingly, allowing YOU to choose when or if to put in rests, or take full breaths of cool fresh air (unwhistled), by making up stuff that uses the specific notes/range/loudnesses you hit with best clarity / musicality in each breathing direction. (Walk up slopes slowly, but stride vigorously otherwise, especially into a wind or breeze, enjoying your infinite-lung-attachment equivalent).
It's great to walk a circuit/jogpath where most (1-2 groups / minute?) are heading one way (like lake Merit in OAKstrdam) at lively-paced walk. If you then walk refreshingly-faster by a bit, your forward-facing whistle will sound like it should, then even if they're talking-while-walking, if you drop-to-mezzo volume for a sec or two/three as you're overtaking them, since your whistling is very forward-focused, coming abreast suddenly means you can belt your heart out without interrupting them, and they may notice that not all whistlers are noise, if they hadn't yet (most in this country still are, I think).
If you're not ready to whistle like you're more competent than the truth, all the time, yet, thus belying simile and forever (co-)owning the stage, you can be an "obvious beginner/student" by stepping over to a tree/dock/tentless-alley/ally-tented unerpass and loudly doing Sound-of-Music doe-a-deer or scales-arpegio-warmups while facing even peripheral-vision fully away from passers-by. If you hit a foul, speak your aside like you mean it "EEew! Dude!? Whatever; try again [whistle-over of same bit]"
Otherwise, if the spectrum doesn't dictate a specific other tactic (which I do REALLY do, really hope you'll share, if it does, once you explore for it, and can explain) then the last point for this morning is my same "ULTIMATE KEY for fast improvement as when learning/performing any new=foreign instrument or language/subdialect (haven't tried dancing application, quite yet, but this, with all due humility, has made me a total-kickass on a thing or three):
**RETRO-JUSTIFICATION: False notes aren't false if noticed as they fly by; they were/are the other hemisphere (or same one, or subneck-soma, or outside world) interrupt-joining suddenly (maybe half-assedly/timidly) by taking or almost-taking an uninvited 1-beat turn: SO TAKE/REINTERPERATE IT AS e.g. them having given/beginning a new pattern, for you two now to go back and forth with, maybe bluesy-bend vs no-melisma-standard turn-taking? call-and-response alternations? or whatever, making the "mistake" into "oh I guess that's what he/we/you was/is/were trying to do" to weave into a duet section or as a transition to the next number.
If you were learning EnglishUSyntax, say, and you were asking (someone you're meeting randomly) "how come you[ to ...the Tenderloin]" in your mind, and translating too-literally from your native-otherlang grammar (or from not-yet-fully-unlearnt Shakespearean-Japanese-HS-English misteachings), and you'd find you'd blurted it almost all out, like you keep trying not to; well... just use the opportunity to get the same meaning out while building agility/repertory-versatility e.g. by ending with "didn't tell me why came to the Tenderloin-- I mean, it's not an obvious place to come, right?"
Great question!! For humming, only obviously unconscious silly happiness sounds good, mostly.
Hope you join us other whistlers someday, somewhere, since I never got a chance to jam much with others whistling too. (Over the rainbow? Good basic tune, too ;-> )