well a lot of stuff, but something that helps me keep up muscle is when im sittting on my couch watching tv i simply get on the floor and hold a v (which is holding your legs up with your lower abs and thighs, so your body looks like a v) and that way i can still watch tv :) lol also run or walk a lot. it will amaze you how much muscle youll gain by going on an hour walk every day. it helped me :)
Resistance workout at least 2-3 times a week, continuing to challenge yourself with weights and switching it up. Aside from that, just stay healthy, eat well and avoid all of this protein crap. Have patience and you'll get there, muscle toning is a process.