I do the tree one too, and with a glowing core of light/strength, up through the soles of my feet, right up the middle through the long bones and the major organs, through neck and head - basically that joins all the chakras - continuing up through the crown. Awareness of the energy connecting above, and then above
that... (though I know this isn't the grounding bit, this is the other direction
) ... and connecting below, feeling how deep it goes, and then deeper... all one length of brightness without division, not really any point where I end or begin but I'm like a bead on the cord.
Sometimes the branches and twigs sprout and I end up covered in rustling young leaves.
Someone introduced me to a meditation where you continue on down and reach, eventually, a wide and bottomless lake down there, which is the source. Very powerful.
It is important to balance the two, not focus entirely on ground or on sky but to make sure you're connected fairly equally with both. It is ok to do one at a time, but over a week or so, you can lose touch with the "other end" as it were and then over-grounded can lead to depression, while over-airy can leave you spinning off in a stressy fluster or else dreamy, head in the clouds, either way not able to get on with things very well.
Another lovely visualisation is of the land, the ground, the hills... as Mother. You sink into them (I do this one lying down) and she embraces you. It is so calming and strengthening. She's warm, and soft in a solid way, you feel absolutely safe and peaceful. After that the rest of the world, I mean everyday life, is back to being pretty small compared with the universe. This one's good if you have trouble getting to sleep, too.
Generally, though, I try to do little bits of grounding whenever I've got distracted and find myself wasting time, or get angry with what's on the news, or being petty or "not myself" talking with someone. Just thinking about my soles on the ground, even if I'm upstairs and in shoes! It's hardest in a moving vehicle, but with a bit of focus can still be done if needs be. This is only to tide me over, of course, not instead of properly giving it attention.
Stamping, forcefully, gets the energy flowing "harder" and helps when it's difficult to make that connection. Your feet really start to tell you that this is the bloomin' ground down there!
In a parenting book I remember a tip for staying in control when embroiled in a row - in control of yourself, not controlling the other person! Grab a wall or doorpost or the stair rail, anything, even the back of a chair but something that can't move is better, and feel it making you steady, and think about your feet firmly on the floor. Say out loud that you're doing this and why. It helps settle things when there's too much fire and air in the room. (When I was reading this kind of parenting book I didn't know the term 'grounding'!)