Inasmuch as I can stay on pitch, I learned to do it by singing "Just like a Woman" in the shower. The intervals were just enough to challenge my then almost nonexistent "ear."
Cosmically coincidental that this should come up because today for the first time I can remember I overheard somebody else shower singing--neighbor lady doing a Boz Scaggs (?) song which I think is called "We're All Alone." or whatever. It's been too long. I've heard worse at open mikes.
I always thought "Non Dimenticar" (sp?) would be a perfect shower song, or "O Sole Mio." You need all those nice Eye-talian open vowels for resonance.