I was looking through some of my old landscape photographs today and noticed something a bit different about a few of them - they didn't contain any sky. It's not something that I've ever seen suggested or recommended in photography magazines or on blog lists on how to improve composition. In fact, in most landscape photographs some sky will be included (I just googled "landscape photos" and all the images that came back on the first page had a fair chunk of good old sky). If the shot is zoomed in to capture detail, eg. in a photo of a stream, then it's more likely that there will be no sky, but it's unusual to see a wider landscape shot without sky.
My tip is to give it a try! I'm certainly going to make sure I look for more opportunities to practise this next time I'm out and about shooting. You do have to zoom in a bit (most of my photos were taken with a focal length of between 70mm and 200mm, with a telephoto lens), and it helps if you're taking the shot from above. You'll need to point the camera down a little, so it's not going to work in lots of situations, eg. if you're on a flat beach. Obviously it will work far better if you're shooting from a high vantage point, like the top of a hill. The photo below was taken from Hay Bluff in the Brecon Beacons, where there's a fine view over the landscape below.
I've included a few of my sky-less landscape shots here. Mostly I captured the shots without the sky, but occasionally I'll cheat by cropping the sky out of the photo afterwards (this can be a good way to rescue a photo where an overblown sky otherwise ruins it).
I think that this method works particularly well if the shot has been taken from a good distance away and where there's something to give it a sense of scale, eg. some people, a vehicle or a tree. It can also work with aerial views of cities taken from a high spot to give a sense of the density of the buildings.