I've used a few fun things to encourage sitting development. I've worked with babies for the past 10 years and I found it helpful that when they are practicing independent sitting to try a few exercises.
First, use your lap/legs to support them and put hands on their trunk to help get them used to this position. Also, use a boppy pillow (in typical fashion) and another one turned sideways on top of the other one so that their arms and trunk can rest comfortably on the other boppy cushion and build the core muscles.
Something else I've done is to use a big exercise ball that adults sit on, but help your baby to sit and rock side to side so they get used to shifting weight and work their core muscles while you keep your hands on their trunks.
I further believe that sitting strength is built upon building strong core muscles from tummy time. This is essential to helping babies grow and develop because without core strength, sitting and crawling and walking can be affected.