Knowledge is Power - Gather Information
The more information you have at the outset the easier your lawyer's work will be. Access to this information is vital to proper evaluation and resolution of a divorce case. Gather records for everything in your financial life, such as: tax returns with all attachments, bank account statements, investment account statements, 529 or college plan statements, records of CD's or money market accounts, 401(k) statements, pension plan documents and statements, insurance policies, and declarations pages (auto, home, life, disability, health, dental), loan agreements and related statements, credit card statements, bills (utility, medical, dental, etc.), appraisals for any property such as jewelry or artwork, and business records for your spouse's business.
Get Control - Secure Your Income and Assets
Make sure your income is secure. Open a separate bank account and have your paychecks deposited into it. This will prevent your spouse from grabbing your last paycheck upon the filing of a divorce or worse. You may continue to transfer money to a joint account to pay bills, etc., but at least your source of income will be secure. This is especially important with auto-deposited paychecks. If you have an account with separate property such as an inheritance, make sure your spouse does not have access to the account (account numbers, passwords, etc.) as well.
Stop the Bleeding - Secure Expenses
Cancel joint credit cards. You and your spouse will still be required to pay them off, but at least your spouse can't run out and rack up a huge debt against the card. Your spouse can still use an individual card (and please note those expenses made during the marriage may still be considered marital debts) but you will at least have a clear division between expenses that is much easier to sort out. Also, if you have depended on your spouse financially, you will begin to establish or improve your credit rating by having an individual account. Even if the credit limit is initially very low, proper use of the card and prompt payoffs will help your credit.
Keep it Top Secret - Secure Communications
Get your own computer if possible and password secure it completely. Best if your spouse has no access at all to the machine physically. Computer programs exist that track your keystrokes, take screenshots of sites visited on the internet, etc. Assume your spouse is watching and act accordingly. This will protect communications with your lawyer by email as well. Change passwords to free email accounts, online banking, cell phone billing, credit card accounts, etc. to prevent snooping by your spouse. Finally, get a post office box and redirect your mail to it. This will also prevent snooping into your mail.
Secure Personal Property
Make sure expensive jewelry, heirlooms, coin collections, etc. are secure, perhaps in a safe deposit box, friend's home, etc. Better safe than sorry - your spouse could sell your coin collection or your diamond earrings on Ebay, and you would be left arguing about the money earned from the item when you really wanted to keep it. It may still be considered marital property subject to division by the court, but isn't it better to have the items and at least have the chance for a rational discussion about it?