Play your scales and do a TON of roman numeral analysis for pieces in different keys
Also play your ii-V-I's or your circle of fifths if you're adventurous in every key.
I agree, of course, since you're right.
But, as a point of interest to me, what exactly IS the best way to mark in roman numeral analysis tunes which have a LOT of changing key centers? Temporarily speaking in jazz-mode, there are so many tunes everyone knows that can/should be played in all kinds of different keys, and lots of jazz teachers insist "every tune: every key" and "every melodic fragment/lick: every key," which has always seemed kind of pedantic to me, even though a good player can actually play just about anything in any key (I'm still drawing the line at blues in B natural, but I'm old school!).
Just a question.
Beyond that, I agree with everyone above: just know all your scales, chords, and basic progressions. Take two or three minutes every day and play some folk tune or something in a "strange" key, like "Happy Birthday" in B major, or "Waltzing Matilda" in F# major, or whatever.
I'll take the outside view and say that, while it's nice to do the fifths/fourths circle/cycle, and certainly sounds more "smooth" while practicing, I prefer to just go up by half-steps/semitones. Personal preference, I guess.