A Lhasa Apso's coat seems to fall naturally to either side and form a part down his back. If you train your Lhasa's part to form a completely straight line from nose to tail, you will enhance his appearance for the show ring.
Different people have different methods of parting their Lhasas. Some like the dog to lie on his stomach while they put in the part. Others (me, for example) prefer the dog to stand. It's a matter of preference. The important thing is that the dog stand (or lie) straight and still. It makes putting in a straight part a lot easier.When I put a part in, I stack the Lhasa on a table and stand directly behind him. Using the end tooth of a metal Greyhound-style comb (some people prefer to use a knitting needle or end of a rat-tail comb) and beginning at the base of the dog's neck, I run the tooth of the comb straight down the spine, allowing the coat to fall to either side. If the dog is standing straight so his spine is straight, there's a much greater chance of getting the part straight the first time.
Once I have this basic part in, I sight down the dog's back from both front and rear to make sure the part runs straight. If it doesn't, I restack the dog and start over. (If I am working on a younger dog that doesn't have the patience to stand still for long, I usually call for some assistance!) If the part is basically straight except for a place or two, I work with those spots a few hairs at a time until it is straight.
If I am training the part in a young Lhasa's coat, at this point I spray the coat along the part with a light hairspray or dab it with hair gel to keep it in place. At a show, I use a light gel or hair spray and mist the coat along the part to keep it in place. "Setting" the part this way is necessary because, as anyone who has ever parted a Lhasa can tell you, the first thing the Lhasa will do after he is groomed is shake.
Once the back's part is set, I begin work on the head and neck. I part the hair evenly on the muzzle; then run the end tooth of the comb from that part, between the eyes, over the head, and down the neck to meet the part I just put in the dog's back. Again, I usually have to re-do a spot here and there to get the part perfectly straight. When I'm satisfied, I "set" the part with a gel or light spray.
At home, I keep my Lhasas' eyefall in topknots or braids to help train the head part so that at shows it fall automatically into place.
Parting your Lhasa's coat may seem like a minor detail, but it's an important grooming practice that is a must for improving your Lhasa's appearance in the show ring. A crooked part often creates the illusion of a poor topline. It also makes you look like an amateur, haphazard groomer.
I won't tell you that getting a part trained is easy. Many of your first attempts may give your Lhasa's back the look of the "mark of Zorro!" The thickness and texture of your Lhasa's coat and its natural tendency to part (or not to part) are all factors to be taken into account. You will need a steady hand. Your Lhasa will need to stand straight -- and to stand still. If you have these things going for you, getting that part in right the first time is a cinch!
If not, oh well. If at first you don't succeed...