Both for legal strategy, and more crucially for the benefit of your children, focus on the quality of your parenting as opposed to custody. The Courts are ultimately concerned with what is in the best interests of the children. If you want to win sole or even joint custody, you'll need to convince the judge you are a capable parent. Too many parents focus on getting custody, and not enough on what constitutes good parenting both from the Court's and their children's perspective.
If you've been an attentive parent, your attorney will assist you in documenting it. On the other hand, if your spouse has genuinely performed most of the day-to-day parenting responsibilities, you will need to learn more parenting skills and prove to the Court that you can make a significant contribution.
Be an active parent. Earn the right to custody by being a good one, rather than through legal maneuvering and courtroom battles.