The trick with using textures is to blend them seamlessly into the rest of the painting. You know you've gone too far when the underlying form starts to become lost. They should be used subtly.
One thing I do to figure out if I've gone too far is to squint at my painting. The textures should be the last thing I make out.
I always paint over my textures. I erase parts out where they wouldn't be standing out, such as in shadow, for example. I make use of varying the opacity if the texture is standing out too much.
One useful technique is to wrap textures around the subject I'm painting. I use the Warp Mode in Photoshop's Free Transform tool.
If you're only using texture in a certain part of a painting, keep in mind that detail and complexity will draw the eye, so use it at the point of interest. Most importantly, textures should never be placed and left alone. They should mostly be used as a starting point.
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