This is not as easy as it may seem to some. In order to maintain lean mass the body requires two basic things. One is regular application of sufficient tension on the lean mass components. Most people thing resistance training in the most popular form, weight lifting. But this can also be acheived through other methods. Also, adaptation to any one type/form of resistance training will reduce the effectiveness of that training on maintenance or increasing lean mass. So, for this component, changing the form of resistance training regularly (every 6 months or so should be enough) will help to make the training stay effective. There are other ways to improve on this type of stimulus. But I teach this as a CEC course and don’t like to give it all away for free.
The second component is fueling the body. This gets tricky. Your body gets accustomed to the way you eat and the way you train. Altering either one will have an effect on the way your body uses that fuel. But it isn’t always what you would expect. If you want to make changes in the amount of energy stored as fat in your body, you need to proceed with changes to your intake slowly and methodically. If the changes are too drastic or overly focused on one particular fuel source, the body may make up for the deficit that it detects by taking structural material from the lean tissue (ie. loss of lean tissue). Again this is not simple and I teach this for a living so I am not giving it away all day here.
Now you can research this yourself. If you are a science minded individual or good at reading research papers/pubications, you should be able to figure it out fairly well on your own. I am willing to give out a few tips on what to search, but I did a lot of this myself over the years. That time and effort (and sometimes I paid others to teach me, when some of these things were still beyond my understanding) is part of what makes my CECs worth the money I charge. The way I teach is also part of what I feel makes my CECs worth what I charge.
Check out my website, www.hawaiifitnessacademy.com if you are interested.