The first step to protecting your assets without a premarital agreement is to keep individual assets separate from your community property.
With a clearly drawn line between individual property and community property, it will be simpler if you need to divide that property later.
Keep Funds Separate
In other words, if you have money in an individual account, keep it there as opposed co-mingling those funds in a joint account with your spouse. Don’t deposit your paycheck into the individual account and don’t pay for any community debt with the funds in that account either.
Keeping Property Separate
Similarly, if you own property that is titled in your name and you want to keep that property as individual property, then pay for all expenses with your individual, pre-marriage money. If you pay for any expense related to the individual property with community funds, it may be difficult to prove later that the property is not community property or that it has a community interest.
The one exception here is if you use individual funds as part of a down payment on a home you share with your spouse. The courts will allow you to get the amount of the down payment back, but will not allow you to claim any additional appreciation for your separate property contribution.
Using Trusts to Protect Assets
You might consider creating a trust prior to marriage. In this situation either you or your spouse may make a clear determination to keep some assets separate. It is more difficult to co-mingle funds when a trust is involved and the purpose will be more evident to the courts should your marriage end in divorce, but you should always consult with an attorney about your specific situation.