Since health concerns should always pre-empt aesthetics, let's discuss trimming between the pads first. The hair between the pads of a Lhasa's feet grows quite long -- and quite fast. If ignored, it tends to mat. The mats become especially difficult if the dog gets his feet wet from rain or snow. Left alone, the mats increase in size and can spread the pads further apart until the dog feels pain or discomfort. If the Lhasa is outdoors a lot, burrs, stickers, and bits of gravel can adhere to this hair or become lodged in the hair between the pads, again resulting in the Lhasa's discomfort. Ignored, mats can cause hot spots between the pads.
Keeping the pads neatly trimmed is an easy task. If you are not showing your Lhasa, you probably will only have to trim the between the pads monthly. With the dog on his side or back, hold a leg steady at an angle that is easy for you to work with. The hand that holds the leg will have to do double duty because you'll need to spread the pads slightly apart so you can trim down between them. You can use a small scissors for this task. I prefer a small, cordless, rechargable trimmer because it is faster to use and I'm less liable to accidentally cut a pad.
Shaggy feet make even the most neatly groomed Lhasa look "unfinished." The unkempt appearance of those feet can be improved simply by trimming and rounding the coat around the feet. Stack the dog on a grooming table and keep him steady. Have someone hold him or use a grooming noose so he won't move while you are using the scissors. Next push the hair up and away from the foot itself and hold it there. Brush or comb a layer of hair over the foot and trim it all the way around fairly close to the foot. Then brush a second layer over the foot, this time trimming it so it's slightly longer than the first layer. Depending on how heavily coated your dog's foot is, repeat this until the foot has a neat, rounded appearance. Do all four feet in the same way.
If your Lhasa doesn't have heavily-coated feet, you may be able to get by with just combing all the hair over the foot and cutting it. I've seen a lot of Lhasa's feet look chopped up when their owners or groomers do this though, so I recommend using the layer strategy.
If you opt to keep your Lhasa clipped, you can forego trimming around the feet; however, you will still need to trim between the pads. Some Lhasas are ticklish or touchy about their feet, so trimming pads and feet is another grooming practice you need to accustom your Lhasa puppy to at as early an age as possible. Trimming these areas is certainly not a difficult task, but it's one that is necessary not only because it improves your Lhasa's looks, but also because it contributes to his health.