I haven't invested enough time into levels to call myself any means "expereinced" but I know you want to make sure you go real slow on everything until you know how everything looks and works perfectly. Go really slow (Same with graphics and gani's) and even your 2nd or 3rd piece could become a masterpiece. Then, once you have perfected your levels by going very slow you will naturally get faster and smoother with it.
Heres how i like levels to look-
Not too much variation
Not too little variation
A POINT to the level
Size correctly. Dont make a small tiny building on the outside have a giant inside.