An old adage of photography is that you should try to create the image you want with the camera rather than relying on post-production. And that's good advice. The less post you need to do on a photograph, the easier your life will be (as post-production throws up an extra set of challenges). But sometimes, for one reason or another, you might want to make your landscapes shots pop a little more by applying a little well-considered post production to them. Here are a few ways you add the drama you are looking for.
Play With the Blacks Slider – Lightroom has a convenient little slider called “Blacks” in develop mode that allows you to have more control over the contrast in your images. Dragging the slider to the right to increase your blacks will usually result in a slightly more dramatic image in landscapes owing to an increase in contrast. Be sure not to clip too much (you can see exactly how much clipping you're doing by holding down the “option” key on a Mac or “alt” key on a PC as you use the slider with your mouse). For a great little video on using the black slider, check out this tutorial.
Give it a Gradient – Another convenient tool you can use to make your clouds and sky a little more ominous is to darken them by way of a gradient. This has a similar effect to using an ND Grad filter on your camera while you are shooting. Sometimes it can also create a more interesting effect if you add a slight gradient to the foreground. This helps highlight the center of interest in the image. There is a nice tutorial on adding drama to a seascape image with the graduated filter in LightRoom here.
Vignette – The final tool that can often be used very quickly and effectively that we're going to mention is the vignette tool/s in Lightroom. Darkening the surrounds mildly while leaving the center of interest a little brighter is an effective strategy to draw the eye of a viewer in any photograph. There is a tutorial on using the vignette palette here. In landscape photography, it can be used quite effectively and looks especially dramatic in black and white landscapes.
This is by no means a comprehensive list of things that can be done to a landscape quickly in Lightroom, but often they can be the go-to tools to spruce up a flat image.