What's the best way to keep a clean trail if you keep joint bank accounts? Do you set up a seperate account for hobby expenses? Develop income sources besides your salary? I've looked the FAQand searched the boards but not found anything on this topic.

just make one off your mutual fund / stock brokerage and withdraw from that. tansferring money to Fidelity or Ameritrade or Charles Schwab doesnt look all that suspicious and you can claim you're just investing more.  

and intercepts statements, goes over the tax forms, etc., then your best bet is to slowly build up a cash dispersement history  for lunches and other misc. expenses, then use some of that money to fund the hobby.

The secret is to not make any sudden changes to it that will stand out.

I was lucky for many years with my ex as she had no interest in looking over anything having to do with financials.   All that changes however when she sued me for divorce, and I had to but heads with her lawyer and CPA.

Start by taking over all the grocery shopping.  Always get 20-40+ cash back over every transaction.  
Set up a separate savings account at a different bank, and make sure it is a paper-less account.  Have payroll deduction of $100/paycheck or whatever you can do and wont create undue questions.  If there are any questions regarding the size of your paycheck, you can always say it's savings for Christmas, birthday prezzies, etc.  
If you do not get checks back (I don't think any bank does that anymore), cash checks for odd amounts not even dollar transactions.

Is there a prepaid ATM card you can buy at target or wal mart?  That seems like it would be easily passed as grocery shopping.  Or what about a cash advance on a credit card, she doesn't care about payments to the card and doesn't review the statement.

Get a divorce.....then u dont have to hide anything.......and she wont be devestated and shattered when she DOES find out......and trust me....she WILL find out.........but this is only my opinion of course.

previous children but...  'my' account is 'joint' so she can get access though she doesn't seem interested.  (We split household expenses...  as long as mine are paid, she has no reason to inquire.)    
When I began this hobby, I began other hobbies which have other expenses...  and she has her own hobbies that I generally don't follow.  Plus I increased my cash pocket money that everyone needs.  I don't carry my full cash allowance on me...  but refill as needed from my home envolupe.  Any pocket cash I don't spend before the next check gets moved to a separate hidden location.  I save until I have time / money / opportunity.  
My bank withdrawals do not match AWOL occasions.

I established an account many years ago to keep my work related travel expenses separate from our joint account. Any funds left over from travel settlements were mine for my hobbies (different ones then).  
 
I rarely travel any more for work so  I just mentioned that I needed money for hobbies and started transferring funds over every pay check. As my pay has gone up, so has "my share". All household income comes from my work so it is not unreasonable for me to take some off the top for my own expenses. I have been doing it for a few years and she hasn't asked me what I do with my money, but I also don't ask where the rest of my paycheck goes. I do occasionally make purchases for my other hobbies and make sure she knows about them. Sometimes I wonder if she is as bad at math as she says or suspects I am up to something but doesn't really want to know what

Him and his hooker can talk food price inflation to break the ice at the beginning of their session. Maybe he can get cash back when he gets his mani and pedi too.

No paper trail.  Some of my regulars have 2nd jobs that provide play income under the table.

Prepaid cards are a hassle: a) because you still have to make sense of the original transactions (what did you need a prepaid card for?) and b) you then have to convert them to cash anyway.

(Yet I skip lunch, or eat light 3-4 days a week).  That cash is now mine.  I quit smoking 15 years ago, but still budget cartons into my finances.  I don't smoke at home honey!  But that's why I smell of smoke when I come home from work (the titty bar near my house stinks of cigarette smoke and I stop in for a beer and say hi to the gals at least twice a week... Although they closed it down a few weeks ago, gotta find a new place to "get stinky"). Going fishing today honey, gonna grab some cash for the fare, the bait and a tip for the mate.  Head out for a few hours to the shore with a fishing rod, a couple of lures and get some sun, pocket the cash.  Going hunting this winter, need more ammo, more arrows, more gear, whatever...  Use half of what I purportedly spend and pocket the rest.  As long as you don't get greedy or try to see a provider every week, there are ways to skim cash, tuck it away in a spot she doesn't even knows exists, never mind will look for.  It's a sneaky way to be, but everyone is happier that way.  Ignorance is bliss for her, plus I get my rocks off every so often.  Win win.  It's been about 26 years of hobbying for me and only three people ever knew that I did, and I was the one who told Them.  A coworker back in '92 who no longer works with me or has any contact.  An ex-gf whom I told before we dated for full disclosure back in '94.  And the current SO for full disclosure when we started dating 19 years ago.  Oh, honey I don't do THAT anymore!  That was before I met YOU!  I was good for quite a few years before I went back to it (about 4 or 5 years in).  So it can be done, safely, quietly, discreetly.  The tough part is having time to yourself when she has no clue where you are and you are not expected back and have no cell service.  Take care of that by doing shift work, sleep at odd hours during the day while she is at work, have a job with odd days off when she is at work.  Cultivate hobbies like fishing and hunting where using a cell is not done because either you're too far out to sea, or being quiet in the woods...

the MAN (not the pussy) of the relationship and be in charge of all the finances. You handle the paying of bills, balancing of the accounts, giving your SO spending money etc... That is the only way to put a little hobby money away for fun without the SO asking questions.

What century are you living in, oral non-connoisseur??

I live in the one where I have no problem with financing my hobby without worrying about it because I control the money.