Firstly, to get really good you need to ski regularly. As it takes many weeks of skiing to get to a level most people would consider to be good, a single week's holiday a year will get you there very slowly. Leaving a whole year between each time you ski also gives you a long time to get out of practice. If you are going to spend more time skiing each year, it makes sense to spread that time around the season, or even the whole year, as much as possible. That way you will have less time to forget between each visit to the slopes.
Consider a second holiday in the winter, maybe to a cheaper resort, or a long weekend ski break as well as a week's holiday. If you are lucky enough to live within an easy drive of a ski area, even a small one, then take advantage of that and go there as often as you can. The cheaper local ski centres are ignored by many in favour of saving up for a big trip to a big name resort, but each has its own unique offering which may be well worth a visit.
If you cannot manage a visit to the mountains then artificial slopes can help keep your skiing on form. A few visits before your ski holiday will get your ski legs back so that you hit the pistes in good form on the first day of the holiday. Most artificial slopes will have clubs with regular coaching sessions so you can keep improving your skiing throughout the year.
Two of the best options to extend your annual ski time are to live in a ski resort for a season, and to get at least some skiing in during the summer months. There are many ways to do a ski season, be it working in resort, or funding yourself to take a year out, but those can wait until a future post as it is a bit late for this season and early for next. Having said that, it is always worth asking around when you are in a resort this year if you want to organise something for next winter, and there may even be vacancies where people have left the resort for some reason.
Skiing during the summer months is a fantastic way to improve, as you will at least halve the gap between your skiing trips. There are many options from this ranging from a few hours on an artificial slope, through a week or two at a resort with summer skiing up to doing a whole season in the opposite hemisphere.
Summer race training camp in Les 2 Alpes
Spreading ski training over the year
Spreading ski training over the year
As well as increasing the time spent on skis, your rate of improvement depends a lot on what you do with that time. I am a firm believer in lessons or coaching in order to improve your skills, as practice alone can only take you so far. Skiers at any level from beginners to world champions benefit from coaching. This does not mean you have to book into ski school for a week. If you already know how to ski then an hour or two with an instructor to fine tune your technique and get an idea of what to work on can be very productive. If you are at a high level then there are many performance skiing courses you can take to improve your skiing in specific areas, be it recreationally or as a competitor.
Lastly, you can improve much faster if you approach all your skiing with this as your goal. Many skiers spend their time cruising about the same pistes, never challenging themselves, spending as much time on lunches and coffee breaks as they do on skiing. There is absolutely nothing wrong with this - it is a very pleasant way to spend time in a beautiful environment, and holidays are supposed to be enjoyable after all. However you will improve faster if you constantly challenge yourself. Ski challenging runs or ski faster (always with respect for other slope users of course). Try to make every turn a good one. On easy slopes set yourself challenges - ski on one ski or do some other exercise, try not to allow the ski to skid, try to make a set number of turns in a given length of piste, or restrict yourself to skiing in a certain corridor. There are many things you can do to push yourself, but it is very easy to let your skiing stagnate if you don't do them.