Is descending hills a skill that can be improved?

Yes it is a skill than can be improved, and it is well worth spending time training that skill.
You basically need to lean forward into the hill's downslope and let your legs "freewheel," so to speak. (Many runners tend to lean back on downhills and "put the brakes on," even subconsciously). When done right you can go appreciably faster than you would have otherwise, without redlining.
Ages ago when I was in high school I read an article in Runner's World about effective downhill running -I'm not sure, but Steve Scott may have written it- that included tips and training exercises. After I put this into practice I was able to use it effectively in x-country races to drop competitors after cresting hills; it made a huge difference, and it works in road races as well.
Good luck.

Running downhill in XC can be a make-or-break skill. Perhaps less so in road racing. Check out the recent race between NAU and BYU or some Foot Locker races for examples. People generally either brace themselves on their tippytoes or bomb down a hill on their heels. I find trying to cycle the legs quickly and concentrate on pulling your feet underneath you works really well, and you start to really rip down a hill without banging your quads. I wish I could find an article to explain it but just check out the 23-minute mark here and look at Luis G to see what I mean:

https://www.youtube.com/watch?v=hPWPgC_uSrk

For training you can do some longer reps downhill to practice that cycling towards the end of the season. Believe Adams State and North Central do this.

Yes, one thing that helps are strides down a slight downhill and relax when running down hill. Avoid leaning back, let gravity takes its course.

A coaching point for this is to lead with the knees and not the feet when running downhill.

Different things work for different people and some people are "naturally" better at downhilling than others, but it's a skill that can be improved substantially. Speaking generally (*many* exceptions), taller people often seem to do better on downhills, which helps offset the advantage that many shorter people seem to have on uphills.

[Realistically speaking, if you're on an extremely steep downhill the "stay perpendicular to the slope" advice won't work and you'll lean back just to keep from wiping out. But you can still "let yourself go" and lean forward on the last 6-10 steps, so you leave the hill with some momentum.]

I absolutely do more than just coast on downhills during a race. I lean forward, expend the same effort as I would on the flats rather than let off the gas, and then take than momentum at the end of the downhill. It's free energy I would never turn down, like employer contributions to a 401k.

Using the downhills to your advantage are one of the little skills that can gap your opponents. Working the downhills, taking a tight turn wide and hard rather than slowing down, going steady up a hill rather than surging, being conscious of the tangents rather than sticking with the crowd, staying under control when going into the wind--these are all little things that could win you the race and less runners do these little things than you would think.

Yes.

A great workout to teach this is to run hill repeats and finish the repeat on the downhill slope. Run up the hill at a normal race intensity, as the hill starts to flatten out at the top, keep the intensity the same and force your self to open up your strides. Fight the urge to rest. As the hill starts to tilt downward, continue to open up your stride and fight the urge to lean back.

Don't do this on a hill that goes into a steep downhill, because you're going to put on the brakes and reinforce the slow downhill running habit.

Be sure to do any downhill running on soft surfaces. Grass is best. Dirt trails are ok. I have seen a couple of runners who tried to improve their downhill running for the Boston Marathon get stress fractures.

I think the key to down hill running is to try to do as much of your mileage on hilly courses. Lydiard always made sure that there were easy days on hilly courses during high mileage phases. Your body will adapt to down hill running just by doing it.

Great advice already, all I can add is to really concentrate on quick turnover of the feet. The poster who mentioned 'freewheeling' the legs had it right, this is what it feels like when I'm doing it right and this is when you really notice the free energy other posters mention as well.

I try and lean into it a bit, really pick up my feet and let momentum feed into it, the general feeling is a very loose kind of controlled chaos haha. When I get it right I very much get the sense of rolling down the hill.

Caution is of course that downhill running is very tough on the knees, if you're unsure or not quite there, take it slowly and in small doses. It does take practise, and some days you're better at it than others

I could absolutely fly downhill when younger. I didn't think about technique, it just felt natural.

Now, as an oldie, I hate downhill running. Running fast downhill just doesn't feel right anymore, can actually be scary on steep xc bits. I suspect it is to do with having less balance and stability (and more fear), than when younger. I suppose I should practice it.

When going downhill maintain a good, quick leg turnover, which may mean shortening the stride a little. I think it was Daniels who suggested running up hills as easily as you could while staying with other runners and then count 50 fairly fast steps when arriving at the top of each uphill run; that's when you kill those who pushed the uphills.