Expect to spend days and weeks, not merely a few hours on a realistic painting. You can't paint detailed realism and also want to knock off a painting every afternoon unless you're painting a small canvas with something simple like a single apple. With this in mind, here are some tips:
Accurate Perspective is Crucial If the perspective is wrong, the painting won't look right no matter how beautiful it is. Get the perspective accurate before getting into the fine detail. Check the perspective regularly as you're painting to ensure it remains accurate.
Shadows Aren't Black Shadows aren't shapes of darker color painted right at the end after you've done everything else. Shadows aren't the identical color or tone in all areas of the composition. Shadows are integral parts of the composition and should be painted at the same time as everything else. Spend as much time observing the subtle shifts in color in shadow areas as you do in the non-shadow parts.
Eyesight Realism Not Camera Realism Don't take a single photo and turn it into a painting. Not because it's "cheating" but because your eye doesn't see the same as a camera. Your eye sees more detailed color, your eye doesn't frame the scene in standard proportions, your eye doesn't have a depth of field that's dependent on a setting. A realistic landscape will be "in focus" all the way to the horizon, not blur out of focus as a photo with a narrow depth of field will.
Color is Relative Color isn't a set thing. It's relative to what's next to it, what kind of light is shining on it, whether the surface is reflective or matte. Depending on the light and time of day, "green" grass can be quite yellow or blue; it's never a simple match to a single tube of green paint.
Compelling Composition A subject painted with great technical skill isn't enough to make a good painting. The choice of subject needs to speak to the viewer, to grab their attention and compel them to keep looking. Spend time considering the composition of your painting, what you're going to include and how you're going to arrange it. Work it out before you start painting and you'll save yourself anguish in the long run.