…when I’m asleep?
I ask because my upper incisors have begun to move forward and apart in the last year or so and since noticing this I’ve also noticed myself waking up with my tongue pressed against them. I fear I may be unconciously undoing three years and a coupl’a grand worth of orthodontic hell. (Mom recently said she ain’t gonna shell out another $2000 just coz I haven’t worn my retainer in fifteen years.)
Now, when I was quite young for my age and quite small for my size, I went to a speech therapist for a lisp (a result of teeth the approximate size of baseball cards, which later demanded the above-referenced orthodontia). After digging through my mental fossil record, I seem to recall her saying that when not using it my tongue should rest up against the roof of my mouth, rearward of the gums.
I never got to the point where this was comfortable, perhaps because I may have a tongue too large for my mouth - a fun, ego-boosting idea in theory, but one I’ve no idea how to prove, not knowing how such things are objectively measured.
Since I never learned where to put it when not using it I rarely stop using it, a fact that (for differing reasons) my schoolteachers found annoying and my wife finds…well, let’s just say she’s developed an appreciation for it. 
To return to the point, if anyone has knowledge, either clinical or anecdotal, of the correct tongue placement AND good ways to make it a habit, I’d be appreciative. TIA