Offensive Flow is affected by the passing and handling of your players, especially your guards and SF. The most effective way to increase it is by training passing/handling on your PG. I don't recommend training handling, as it can be trained with more skills with 1v1 training (I prefer FW, but G is fine too).
It's also important to have good your passing/handling on your SG/SF. You can't expect to have good flow with less than 7 HA/PA on your SG and SF (and that's just for D IV!)