Properly warming-up and stretching your muscles is important before any kind of workout, including all forms of martial arts. The same stretching exercises you do before a practice session are the answer to how to increase your flexibility for Jiu Jitsu, whether you are in a tournament, or just keeping your skills sharp in the dojo.
Improving flexibility involves doing sustained stretches of all major muscle groups, one at a time. The order you do these stretches in is not as important as making sure you give enough time to each muscle group. This is the secret of how to increase your flexibility for Jiu Jitsu, as well as any other physical activity.