I exercise my tongue all the time. It doesn’t make it longer, but it makes it more dexterous and stronger so I have more stamina downtown. Here are a few things I do:
Fast flicks: side to side and up and down across the inside and outside of the lips — inside when you’re at stoplights or otherwise around other people. Outside may require eventually chapstick.
Super fast flicks: flick your whole tongue in and out as fast as you possibly can across your top lip. It should sort of cup at high speed and you’ll get a popping sound. This also really works your jaws.
Extended tongue flicks: stick your tongue out as far as you can, then flick side to side. It’s harder to control the flick up and down, but do that too.
End of tongue flicks: stick tongue way out and try to flick the very tip as you keep the rest of the tongue straight.
Resistance tonguing: using anything that won’t irriate much, like a piece of smooth plastic (rubber can actually be abrasive). Lick it hard, practicing how to widen and sharpen your tongue as different positions get fatigued and need rest.
Timed flexes: stick your tongue way out in a point, or wide, and hold it there for 30 seconds to a minute. Do as many reps as you can.
Gum: stick like four pieces in your mouth and chew. Strengthens the jaw too. You’re going to need strong jaws. This is also an exercise for singers.
Jaw fatigue is an issue after you’re down for a long time. You need to change technique every once in a while relax the jaw, or the tongue, or whatever has been used too long. Lips are nice, as are well lubed and careful fingers.
And I guess there’s also the old reliable method: eat a lot of pussy. Kissing will do too.