I agree with above first you need to learn how to grow plants. That certainly makes scaping a LOT easier.
For me the first thing you need to is have a plan. And that's not as easy as it sounds. When I decide to change the scape around I spend a LOT of time just staring at the tank. Try to imagine different placements, try to figure out what plants might fit in well, try to determine what colors/leaf shapes I am missing, etc.
Part of that planning is knowing how different plants grow. Some grow straight up, some twist and turn, some easily create a nice bush, others need a lot of attention to get the right look. You also need to consider speed of growth. When you see a pic of a tank it is a moment in time. What you don't see is the careful planning during the trimming process. Getting an entire tank to peak heights for the scape takes forethought. Again it takes a lot of practice and trial and error.
Like Art said leaving some spacing between the groupings when trimming is important. Things will fill in and will look more natural over time.
I've said this many times but try to stick to the plants that like the soup you are serving. Keeping 20+ species happy at one time is tricky. Different plants have different optimal parameters. Some plants will be almost impossible to keep together in peak health. Better to let the ones go that don't behave than trying to chase parameters to please them.
It helps to be picky with plant selection. Try not to be a hoarder. I know that is easier said than done. When I look at my tank I try to think in terms of what I am missing. A certain color? Certain leaf shapes? Try not to buy something just because you see it if you have no plans on how to use it. This is why I don't dabble much in rares. Many of them really aren't that good for creating a layout. If it doesn't have a place or fill a void in my vision then I try to avoid the temptation to buy them.
And then once you get all those things figured it out you need to have the desire to practice good trimming/horticulture. This takes time and patience. When I perform a big trim it can take several hours. Many times I am trimming large groups of plants one by one. I find the process therapeutic and look forward to it. If you don't enjoy the process, then the hobby might not be for you.
You'll know you got it bad when you get done with trimming and then pull a few groups again just to move them over an inch. I've done that more times than I care to admit. I always seem to have this vision in my mind of what I want the tank to be and am constantly trying to get it there. And while I never feel I got it exactly right, I keep on trying! I think that's what has kept me in the hobby for so many years. I always feel there is room for improvement and have the hope that someday it will all come together exactly as I planned.