Yes, as hamster pointed out, certainly don't use any credit cards that she has access to the statements of.

personally, I put a password on my phone and refuse to tell her what it is even though she routinely tries to figure it out or begs me to tell her. I also keep a separate bank account. We had the same account for a while but I got sick of her always knowing where I spend my money, so I put a stop to that. One thing I have started doing recently which works pretty well is that I have stopped telling her where I'm going ever. Even if I'm just going to the store or going out with an old friend or whatever, I just refuse to tell her any information about where I'm going, who I'm with, or how long I'll be gone. Therefore, it's not overly suspicious if I don't tell her where I am or who I'm with because I never do.

Another great idea is to turn the tables on her. Find something that she does that is wrong and she knows it and always change the subject to that whenever she tries to confront you about anything or get upset with you about anything at all. It points the finger back at her and when confronted with having to face her own inadequacies, she'd usually rather just end the discussion. Actually this is an old tactic women have been using on men for years.

The problem in your situation is that you have been married so long that suddenly changing your behavior may be suspicious in and of itself. But if you use a little creativity, you can likely come up with something that will work in your situation. Just remember that most people are ruled by emotions and are therefore easily manipulated.