As Feo said, deep breathign helps a lot. The more you do it, the easier it is to tap into it.
But for what it's worth I used to have to do yoga for about 20 min THEN I could do meditation. If not meditation was hellish as i just had some juice I had to use up.
Now, meditation anytime is no issue, but when I was wired/tired and unable to sit for too long it helped a lot. But only AFTER yoga.
We're all different, but maybe try that.