ALWAYS ALWAYS ALWAYS pull your clipper away from the body when nearing the skirt line. Just barely pull it away from the body. DON'T EVER just stop, that leaves lines... Also like said above, remove as much undercoat as possible. I had to fix another groomers westie today because she didn't listen to how I told her (or showed her umpteen times) on how to do a Westie properly.
You can also use thinning shears to help blend the skirt. I personally use the Furminator, the comb and the brush, pull the blade away, use thinning shears and never use anything shorter than a #4 blade. In some cases when the owner wants it shorter I do it, but I always warn them, that it's harder to blend.