1) Start small - Pick one thing. A new class? Or a mini yoga practice at home that you can do every day just for 10 minutes (just pick 3 or 4 yoga postures and do that for a couple of months). Don't choose both of these, just choose one thing as that is effort enough. Focus on getting the habit started, rather than the results that you want to get from doing it.
2) Be realistic - pick the time that you can do every day or every week that is sensible and that can work for you more often than not. Then pick a class or a short daily practice, 3 or 4 poses that you think you can do and that you will enjoy. Your focus should be on getting the habit ingrained. The rest will follow.
3) Get started - the main thing is that you do it. It doesn't matter if you do it well, or if you have a cold so need to go gently, or that it is raining and cold and you don't fancy going to your class. The main thing is to get started, do it, and do it in a way that you can stick to. Get yourself on your yoga mat every week or every day.
4) Tell someone your getting started - this will help you make the commitment and increase your likelihood of doing it!
5) Don't miss two in a row. If you need to miss one, then ok. Try not to miss two in a row as then the habit is broken and all your good effort may not come to fruition. Keep going, even if only gently if your under the weather. If your away, then try to make up the class or the practice in another way that still counts.
Good luck with getting your new yoga practice in place. The benefits will be worth it, all you need to do it turn up and do it :-).
Happy new year!