By the time you have enough confidence to ski in untracked snow, you have come a long way. You can continue working on your technique and correct it as you go along. Here are two things that you must focus on.
1. The stance is the same as the one that you would use on the marked ski runs. It is a common misunderstanding to think that your should lean back on the skis when skiing untracked snow. in powder you should maintain a position on well over your skis. If you do not, you will lose control and your skis will run from you, just as they do on groomed terrain. Focus on standing in centre of the skis and using your arms to keep your balance. Hold your arms out from your body and in front of you. Remember to use your ski poles. They are a big help when practicing your rhythm and helping you stay forward on the skis.
Use your arms and poles to keep balance.
When you get serious about powder, you might want to consider getting extra large baskets for your poles. The baskets are the plastic plates at the bottom of the ski poles and with extra large baskets, you can prevent your poles from sinking too deep into the snow.
2. Use your skis in a different way in powder. It is clearly an advantage to ski powder with your skis together and your weight evenly spread on both skis. this way, the skis can move more easily through the snow without getting caught up in the snow mass. It also prevents one ski from sinking deeper into the snow than the other.
Keep your skis close together.