Many times the consistency of frosting will vary depending on the humidity of the room, the moisture content of your butter, etc.
For frosting that's too thin and runny - Add ½ cup more confectioners sugar while beating, then test the frosting again. Continue adding ½ cups of sugar till the frosting holds its shape and can be spread or piped onto your cake.
For frosting that's so thick you can't spread it smoothly or pipe it from a bag - Add milk, 1 tsp. at a time while beating till the frosting is smooth and creamy and still holds its shape.
If your frosting looks like it's separating or splitting, it means that your kitchen is too warm, or that the butter got TOO soft. To correct it, STOP beating immediately. Cover the bowl and place it in the fridge for 30 minutes, then add ½ cup confectioners sugar and rebeat the frosting. Most times this will fix the problem and produce a lovely creamy frosting.