I'm sure this has been addressed before but I would appreciate any thoughts on the subject. I'm far from testing for my 4th Kyu but it will come with continued practice. Suwari waza is tough on the knees.
Figure out what muscles are tight and/or weak, and take some time to warm and stretch them every day. use a foam roller or the like if they are particularly tight.
Practice sitting in seiza. You might want to practice squatting also, that seems like it has helped me.