The best way to do this is to give you an example, simplest of which would be combining bridges and dirt. Observe:
See the tiles between the dirt and the bridge pieces? Those tiles are located at either end of the bridge doodad. By looking for what dirt and bridge tiles both blend with, i created this.
To put it simply, the best way to blend is to look for what goes together. When blending high and low dirt, (not the cliffsides mind you) i used pieces from the ramp that connected the two dirts. Seen here:
(Note this was done quick for this paticular post.)