One of the most crucial skills you need for thermal- and XC-flying, is to have control of the roll-pendulum. Roll-pendulum is necessary to make the glider turn in the thermal. The thing is that the more steep bank on the glider you have, the less turn-radius you get. For thermalling there is no such thing as a flat turn. Everything is about to stay in the strongest lift and to avoid the sinking air as much as possible. More bank on the glider give you very little extra sink, compared with the extra you gain when you stay in the core. So when you add up you will get up more effective and spend less time in the thermal before going on next glide. When going for XC, time matters. Usually the day is limited in flyable hours, and the speed is all we can work with to fly as far as possible.
It is not always about being effective and fly fast. Sometimes it is more important to just stay in the air. If the thermals are marginal, it will also gain to stay in the strongest lift or to avoid the sinking air around it. Or to stay in the less sinking air!
Small thermal. If the thermal is small, you need to have pretty much bank just to stay inside the thermal. If your turn-radius is too big, you will experience that you fly out of the thermal and into the sinking air around it, and then back into the thermal. This make you fly through the turbulent air over and over again, and it is not very effective. Now, it is better to get a steeper bank once you are inside the thermal. The air is more calm, and you will get up faster, when avoiding the sinking air around.
Big thermal. When the thermal is bigger, you can fly with less bank and still stay inside the thermal. But in terms of being effective, it is usually better to have more bank when you are in the strongest part of this big thermal. You can say that now you want to avoid the parts with less lift. When you are inside a big thermal, it is more wanted to search for stronger lift inside the thermal. So don't just make 360's, but open up and fly straight when you feel that your lift increases. When you feel it decreases, you should bank over more again. This way you can manage to stay in the core.
How much bank is recommended? When banking over more, the turn-radius will decrease. So will the time you spend doing every full turn (360). By measuring the time for every full turn, seconds will be the measuring unit for how much bank you have. An average full turn when flying thermals, is 18 seconds. Variety to be between 12 and 30 seconds. When you are searching for better lift in a big thermal, it can be a lot more. But if your average turn is 30 seconds or more, you better bank over more for a better experience. Get an instrument which is showing seconds with big numbers, and get out and measure your bank.
Roll-control. To introduce the roll-pendulum you can use weightshifting or your steeringhandles, or a combination of these. Which method you choose, depends on what you feel is best for you. It can also be done using the backrisers if you like. Be aware so you don't spin your glider when introducing the roll. In the beginning it is better to start with less bank and then increase it when the energy gradually increases. When you are more experienced, you can be pretty hard on the steeringhandles to make a sharper entrance to the bank. Once you are in the turn, the use of the steeringhandles and weightshifting, is only to maintain the wanted bank of your glider. Since you are in the thermal, and that you want to search for stronger lift, you need to be in control of the roll-pendulum at all times. You need to meet the changing air which constantly try to affect the glider in roll, pitch and yaw. This require you to know the dynamics of your wing. The best way is to fly lots of thermals. But practicing swingovers (not higher than 90 deg), will help you understand how changes in roll, yaw and pitch act together. If you go to a SIV-course with an acro-instructor, you can practice high wingovers or asymmetrical spiral. (Do not practice WO by your self, it may put you in a bad position). This way you can learn to fly with the glider and not have to force it in to the thermal, when the thermal try to spit you out. It is also recommended to practice 360s with measuring the bank, to learn to know how much bank is related to different duration of the full turn. Practice also to exit the turns, with no pitch, like you did on the spiral when you went to SIV-course. If you bank over too much, your wing will enter a spiral, but it is surprisingly much bank that is required for that to happen. Flying in thermals with other pilots is more fun, and it gives you direct feedback if you are in control of your roll-pendulum.