When you get a first (if ever) yodel make a great fuss of your Basenji. Get very excited about it. Once a yodeller realise how much it pleases you they'll continue.
Still that doesn't solve it if its a Basenji who never yodels! We've sometimes managed to get a Basenji to yodel by playing a certain note on guitar but I'm not sure which note as its my husband who is the guitarrist.
Mine baroos at people and traffic as we're walking and at the dog park full volume. Most people laugh. Yesterday he baroo'd at this lady across the street pushing a baby stroller and walking her dog and she yelled "a singing dog"! He never makes a sound at home though only when we're out.
I would love to have Roxy yodel when I ask her to…
... instead of when she's dreaming in the middle of the night as she sleeps next to me.
Otis was a very quiet B as well, I onlyt heard hom yodel twice and even he was shocked when that happened.. I tried everything to get him to speak..but nothinmg worked..oh well. If you figure it out, I too, would love to learn about it..:)
I've got a quiet B as well. I get the "musical yawns" on a daily basis, and every once and awhile he'll make this chortle (growl/bark/howl) when he's really excited.
I try to motivate him to yodel with no success. 
Shaun is a pretty good baroo boy. Everyday when mom comes home or when he is enjoying himself.
The second video on this page has some good baroos:
http://basenji-shaun.com/video.html
Nice choice of soundtrack music on that vid, SeattleMax – St. Etienne! 
Well, I don't need Bowpi to baroo on command... But maybe I feel compelled to keep playing these barooing videos for her to see if she'll spontaneously join in. Like training a bird to sing? Heh.
It's okay if she never yodels. It just seems like there's some threshold of excitement where a baroo can't help but come out. But she's not very excitable, period... though she often gets into a crazy sprinting mood at the park. So there's no doubt as to her basenjiness. 
Hope baroo's everytime we come home and wags her little tail. She also drops/folds (?) her ears when we first walk in whilst wagging her tail and then baroo's and then licks us to death lol. We love her warm greetings every time 
Anyone else's B's do the ear thing? Ill try get a photo of it.
I have 5 basenjis. Nicky, 12+ years old, has yodeled exactly 4 times in his life. Once at my mother in law's house when he was just so excited to see me and my husband come home that it was out before he realized it and he seemed startled that the noise had come from him. Once on walk when my husband was playing hide and seek with him while I walked him. Once for his black and white son Clark. Once for son L'Ox.
His housemates are all exceptional yodelers and have even been known to hold apparent "yodeling contests" just to show off their yodeling skills. No matter how vocal the others get, he does not join in even when they are rewarded for it. It is just not his thing, which is too bad since he has a wonderful voice.
My Red boy, Congo, had an awesome BAROO! We were so happy to hear it every time! He couldn't do it enough for my amusement! I don't remember at what age the BAROO's stopped but I'm thinking 14 or 15. When he was in his prime, I think he yodeled at least once a week.
Bandit, my 7 year old tri girl, has never BAROOED in her life! She purrs like a cat and cries like a newborn baby, but never a yodel.
Hunter, my 2 year old BRAT tri, has a great yodel. His voice usually cracks mid-yodel. Almost like he was a heavy smoker at some point in his short life! He also had the loudest yawn on the planet!:)
