The first thing to begin with, after your base training period, is to start training your legs turnover rate or "cadence" on flat ground. You aren't trying to do this over steeper terrain, purely flat, or a slight uphill. Using intervals of 20 seconds, and extending these as you progress, to train is the first step. By training on flat ground your body is learning how to turn each leg over faster, and by slowly increasing the amount of interval time, to maintain that faster cadence. Flat ground also makes sure you are training this alone, and that other muscles and systems, are not being recruited which may hinder the quality of your workout. On flat ground you are training your forward propulsion, whereas training on hills you are training your vertical propulsion. You must train these separately.
(Photo Above: Jeff Colvin brings vertical and forward momentum into one, 2010 WC Vertical Race)
Since you have to go up in ski mountaineering races, it would be silly to not train your uphill aerobic capacity, so continue doing so. Easy as that. This is your vertical propulsion training. You will not notice an increase in speed right away, training takes time, and remember patience is a virtue. Continuing with these two main training modalities, your cadence will naturally begin to increase, as the flatland training will begin to transfer over to the uphill training.
Now there is a trump card that can factor into this, which may slow some peoples improvement down, and that one thing is powder. Skiing in the backcountry as many of us do, in search of beautiful powder, and bottomless blower does actually limit our training. Yes it does help build power, breaking trail is a lot of hard work, but slows our cadence down. Your body is constantly learning, and breaking trail solely as an aerobic workout, won't stop you from progressing but will slow your progression. The reason is you cannot swing and move your feet as quickly, as if they were in a skintrack. Racing is done in a slightly more artificial ampitheater than backcountry skiing. It is in the backcountry, but there is a skintrack and bootpack put in for you, and although you may find the odd skintrack in the backcountry you are primarily breaking trail. But hey, we can't neglect powder now can we.
Gear is also a large factor when it comes to cadence. Being able to turn your feet over quickly, and efficiently, is hard work. The weight from your skis and boots, are limiting, so having the lightest weight setup as possible allows you to move your feet faster and with less energy. (Photo Below Right: Our room mates boot at Pierra Menta, trying to find more speed with less weight)
Now this is the simplest way you can look at cadence training. There are so many different modalities to speed training, and not just flat. Racing will improve your cadence, as training fast will help you to race fast. A race pace training day is good to throw into the mix as it will sew all the pieces together. Strength training, and cross training will be important as well. Other cadence training workouts can be a 30-40 minute run, and classic cross country skiing. I would recommend that while training with your rando race gear on the flats, that your skins are nicely waxed, so that your glide is trained at the same time. Remember efficiency is the key, and your body is constantly learning, so repeating race day style strides while training will create fast race day strides. If you are turning your stride over faster, but aslo gliding 5 centimeters as well, that equates to a 300m elevation gain over 1000m.
In the end, do what works for you, and monitor your training well. Ski Mountaineering Race training is still developing, and finding new and better ways of training. But to give you an idea on how fast some racers are, top World Cup athletes are climbing at approximately 114 steps per minute (measured from the first ascent of the 2010 WC Individual Course). How many steps are you taking?