Besides the normal vinyl care and feeding tips I have three things.
1) Posing
Familiarize yourself with the internal structure (skeleton) of the Obitsu body. You don't have to disassemble your doll to do this. Look at the cut away picture below.
Even though this picture shows the 60cm three part sliding joint style skeleton, the ball joint skeleton as in the 55cm has the joints in the same places.
Of particular importance are the joints in the torso. There are two ball joints in the torso. This is where the torso will bend. You can squeeze the torso vinyls to feel exactly where these joints are. Once you get a feeling for where these joints are it will be much easier to bent the torso as needed. Many people have broken one or more of these torso skeleton parts and I believe much of this is from mishandling. With resin dolls you can see exactly where the joint is, where two resin parts meet. But in the Obitsu two piece soft bodies (as in the 55cm) the two joints are located above and below the "seam" between the two vinyl torso parts, which is not where the torso bends. Some people have replaced these three parts with the spine part from the 50cm body, which has a more contortionist range of motion. It is also one of the few 50cm parts which will fit in the 55/60cm body. I have numerous 55/60cm bodies and have posed them weekly for MADh Vinyl and have never broken one of these parts. I have also never had a resin body so I learned to pose vinyl bodies, not resins.
When posing/moving limbs is is best to bend or twist the joint one axis at a time, rather than grabbing and pulling the limb into position as you might with a strung body. The Obitsu skeleton is designed with many twisting and hinging joints and it is best to make one part of a move at a time so as not to twist of bend a joint part in a direction it was no meant to go. Bend the joints slowly till you get the hang of it and your Obitsu body will be trouble free for years.
Yes, many joints will squeak when moved. This is normal. It means the joint is tight and will hold a pose well. Do not try and lubricate a squeaky joint. You will only damage the plastic.
2) Consider some parts replacement.
The stock 55cm body has a number of external vinyl parts which were designed for a smoother look. This comes at the cost of some range of motion. Most notably are the joints in the arms (wrists, elbows, shoulders). The vinyl parts were molded so as to cover more of the ball joint, preventing the joint from bending when the vinyls bump each other. What I have done to remedy this is to buy a second part of arms, the ones using the more bendable (but less aesthetic) slide joint elbows. You would need to purchase the 60RP-F01WP-31 Elbow part, the 60RP-F01WS-26 Upper arm vinyl, and the 60RP-F01WS-27 Lower arm vinyls. These three parts are white skin and sold as pairs so you'll get two arms. You would use the existing shoulder joint, wrist joint and hands.
The other part to consider replacing is the chest piece. The stock chest piece (55RP-M01WS-21) is rather plain. Parabox has a limited supply of their custom "macho chest" (AC-PB-5500A) The macho chest has a bit more bulk and muscle definition. Parabox says you need to replace the internal chest part but I'm using it with the existing 55cm internal chest part without issue.
http://parabox.jp/obitsu/slimmuttyo/koukan.html3) The hair dryer is your friend.
All Obitsu parts are replaceable individually. If you break a knee, you don't have to replace the whole leg, just the broken part(s). Many vinyl parts will seem to be nearly impossible to disassemble. This can be remedied by the application of heat from a hair dryer. Heating the vinyl will make it more pliable and disassemble and reassemble much easier.