My newest singing tips video gives you 3 suggestions (+ 1 bonus tip) for how to relax your tongue and get it to stay neutral while you're singing!
Two ways to gauge tongue tension are:
(1) how your voice sounds (do you sound like Kermit the Frog?)
(2) How your throat or neck feels (dull pains along your jaw or at the top of your voice box).
If you have tongue tension or a tense tongue, fear not! 3 easy triggers to get your tongue to chill include:
(1) Thinking "UH" while you sing
(2) Pretending you're drunk (not kidding)
(3) Practicing a "conversational" version of your song or warmup
And as a bonus tip? Don't make an enemy out of your tongue! As a singer, it does more harm than good to be stressed out about the body part that's causing you tension. Instead, think positively about your tongue, remember that it's just trying to help, and eventually it will learn to back off and stay out of the equation.