Stay on the top side, keep your feet as wide as possible, keep your lanyard long enough to be able to move fluidly and stay upright on slight leaners, on hard leans it might be necessary to lean over and use your hands for balance. Sometimes on a real hard leaner I will hook my lanyard with my thumbs so I can advance my lanyard while still using my palms on the tree and walk out on all 4's. When its time to make a cut you can take a double wrap with the lanyard to lock yourself in. Kneel or sit on the limb/lead if needed. If its possible, tie into something above, or a higher neighboring tree. Even if your tie-in point if fairly far away, you can keep your rope tight and it gives you something to lean on and more stability. And of course, with years of practice, it becomes much easier. I used to find a leaner with a twist or a big dog-leg to be very challenging. After 5 years or so of daily climbing it became much easier. Now, its almost all just 2nd nature. I almost never find a situation I haven't been in 1000 or more times and I have strategies for all of them. Experience is king.