I say “jacking off” for men and women.
Isn’t it “jilling off” when you do it? 
Since “merchant banker” = “wanker”, I suppose “merchant banking” would work as English rhyming slang.
Jerkin’ the Gerkin and stainin’ the Merkin!
(For some reason the ‘g’ must be dropped.)
Immanentizing the eschaton.
All the OP needs is a question mark.
Polling: Popular terms for masturbation?
Yes, it is 
This far and nobody’s mentioned “punishing Percy in the palm”?
(BTW popular terms for masturbation in Oxford would be Michaelmas, Trinity, and Hilary.)
One that’s stood out in my mind ever since I read it is “buttering your bialy”.
Variants I’ve heard in the USA:
For the first, waxing the bishop.
For the second, having a date with Rosie Palm(er) and her five sisters.
From the show Angel, we have “taking some gentleman’s time”.
This thread is bit dominated by male euphemisms. I’ll try to give more equal time to the female euphemisms.
Polish the pearl
Dial ‘O’ on the pink telephone
Check the oil
Apply lip gloss
Nulling the void
Rolling the trackball
How can we have forgotten “Plucking the magic twanger,” taken from the Howdy Doody show in the 1950’s.
“C’mon, Froggy, pluck your magic twanger!”
Was that Gabby Hayes or Slim Pickens?
This is probably more of a Southwestern (NM, specifically) one, but a long ago ex I had called it “Peelin’ chiles”.
He particularly liked it because his friends could call his house, not get an answer, ask him what he’d been up to when they called, and not really know how to respond to him saying, “Peelin’ chiles.”
(It may also just be a “my strange ex” thing.)
I once had a boyfriend who referred to male masturbation as “jerking for Mister Goodbar.” Dunno whether or not he coined this phrase, but I haven’t heard it elsewhere.
I always liked “flogging the dolphin” and “beating the seal.”
“Beating your meat” was fairly common in junior high. Most people my age now just say jacking off or whacking off.
I’m 90% sure that’s one of David Letterman’s Top Ten Lists.
Woody Allen once said something like, “What’s wrong with masturbation? It’s sex with someone you love.”
I’ll be in my bunk.
A Heartbreaking Work of Staggering Genius
I think you’re thinking of Top 10 Star Wars Fan Euphemisms For Not Having A Girlfriend. Funny in its own right, and maybe the inspiration for some of those 15, but not as good. 
Thai: “Chug wau” (flying a kite). I’ve always loved this one. The whole “go fly a kite” implication that it’s fun but somehow not the real thing…
I always heard “peelin’ chilie” to mean slacking off, beating off, or sitting around with your thumb up your ass, and not to mean actual masturbation.
And sitting around with your thumb up your ass isn’t actual masturbation?
mm