Make sure that your blade drive is not worn out and your blade is sharp. Westies are one of the most common for clipper lines, because of that thick dense body coat. For that reason, I rarely use a blade on their pattern, instead opting for the Wahl Stainless Steel attachments, usually a #2. You can also card out the back coat before clipping to remove as much of that undercoat as possible. I hope you aren't using a 10 blade on the back because its near impossible to get a nice smooth finish on a westie with a 10 blade..7F reverse works better (IMO that is way too short though) but is hard to blend to the skirt and legs.
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.
I prefer using snap on's too, and don't like going short. I hate the look, and trying to blend a #7 patterned Westie, but some people like it that short over the top, so that is just what you gotta do, and if you're good you can still do it well enough. It just takes alot of practice.
I find that running the clipper straight down and "off'' the body (being careful not to push in once you've reached the skirt line), rather than trying to pull it out, like Raggs mentioned, is an easier way to further shape and blend the skirt, unless you choose, to clip in reverse.
Using a blade in Reverse can help alot with clipper lines, especially with thick coated dogs, so if you like, say, a #4 blade length, use a #3 blade, in reverse, stopping about a half an inch above the skirt line, which you will then use thinners to complete an even blended appearance. The reason I give you such a 'wide margin' is so you have alot of blending space to work with, until you get a real grasp of it, then you can gradually decrease that width, until you eventually clipper right 'to' (which is really, just a hair above) the skirt line, and then blend in.