I have the following mesh. As you can see there are quite a bit of triangles that are unnecessary, and can be reduced. However, if I use the decimate modifier, other triangles are also decimated and morphs the shape.
If decimate is too destructive, then while in edit mode, select all, then press X>limited dissolve to remove excess edges in your model. You can adjust the angle threshold in the operator panel as needed. Don't forget to also remove doubles with W>remove doubles.
You can also try tris to quads. Press Spaebar and type in tris to quads, and select the operation from the menu, or or press Alt + J
Also the decimate tool concentrate the triangles where there is more details. You should smooth with a large brush your entire model too.