Exercise all your time, plus more
If you go to court for a review of your custodial situation, the Court will be more interested in what the parents actually are doing, than in what any prior court orders say they should be doing. Use all your custodial time that you are financially and physically able to use. If you can accommodate the other parent's scheduling by offering to have the child(ren) with you, more than the order says you "must", do it. This is not "baby-sitting", it is spending time with your kids.
Exercise your rights (and responsibilities) beyond the time-share: Education
There is more to parenting than whose house your child sleeps at, on which nights of the week: Request scheduling information about all your child's school functions, and attend them whenever possible. Whether or not the other parent provides you with copies, take independent steps to arrange with your children's school to receive scheduling information, report cards, etc. If you receive any of this information, and have any reason to think the other parent did not, send a copy. Know your kids' teachers. Make sure that your kids' teachers, and school personnel, know you. Make sure you are on all parental notification, and emergency notification cards. Keep your notification information on those records absolutely current. If your child is having any sort of trouble in school, find out from the school (not just from your child or the other parent) what is going on, why it is happening, and what can, or must, be done to fix it. Then, as much as you can, do it.
Exercise your rights (and responsibilities) beyond the time-share: Health Care
When (not if) your child needs health care, find out what is going on, whether it is a regular dental check up or physical exam, the treatment of a cold or earache, or an ongoing course of treatment for a serious chronic condition. There is no better way to stay informed than to take your child to the doctor yourself. If you can't, ask the other parent regularly for information. If you have any questions or concerns, discuss them directly with the health care provider. The other parent is not your best source of this information; your child is an even poorer one. "Health care" does not just mean your child's doctor. It includes the dentist, the orthodontist, the chiropractor, and any psychotherapist or counselor. Know who is paying for health care, who's carrying the insurance, and who is entitled to reimbursement. If it's you, ask for it promptly. Keep records.
Decision-making: use it or lose it
Even if you do not have joint legal custody, under California law you are entitled to information about your child's education and health. As a joint legal custodian, you have both a right and a responsibility to keep informed about, and to be involved in, decision-making. If you don't participate in decision-making, the Court may conclude that you have no further interest in having a hand in decision-making
BE APPROPRIATE: Behave, in all of your interaction with the other parent, as if everyone was watching you; they are.
Assume that at some point in the future the Court, and any Court evaluator, will consider both the content and tone of your communication with the other parent, and the other parent's new spouse or significant other. Not only shouldn't you use your child as a telephone ("Tell your mother you can't spend the whole weekend next weekend....") you shouldn't use your child as a mail-carrier.
BE POLITE: Behave, in all of your interaction with the other parent, as if everyone was watching you; they are.
Send nothing to the other parent you wouldn't want the judge to read now, or that you wouldn't want your children to read, twenty years from now. Even if the other parent is acting like a jerk, don't act that way. If scheduling changes need to be made, give more notice than the minimum amount required, whenever it is possible. If you are running late, even a little bit, call. If you need to make or change your child's logistic arrangements (different clothing, school books and materials, sports gear, etc.) communicate with the other parent the need for those arrangements, IN ADVANCE. Confirm your communication with the other parent by short, polite notes or emails . Do NOT include any discussion of the other issues of the case, or of why the relationship failed, blame for any past events, etc. These notes should be the bare MINIMUM number of words necessary to convey the information, plus plus "Please", "Thank you", and "You're Welcome"
Your children are not the litigants; don't treat them that way
It is appropriate to discuss resolution of your case with the other parent, or between the attorneys. Children, however, are not litigants. They have no obligation to settle their parents' cases. They should not be burdened with any discussion of the financial aspects of the case, ever. Your children have no obligation to tell you their preferences regarding custodial arrangements. They do not even have an obligation to have such a preference. Your children should not be obliged or expected to decide their own custody arrangements, although you should consider their wishes.
Remember: this is your custody case, not everyone else's.
Unless there is no appropriate way to communicate directly with the other parent, don't use anyone else as a messenger. Your relatives, your new spouse or significant other, the other parent's relatives, your friends (mutual or otherwise) and your child's friends are not obliged to take sides, or act as tie-breakers. Don't ask them to. Don't expect them to.