Aside from someone turning you in or you do something stupid like telling the IRS that you were stupid, look at it this way. Now days you can't buy anything of real value without a paper trail. So if you are not reporting income, you only option is to pi$$ if off. I'd rather have my money working for me. A guy here with laundromats, had 3 sets of books but put the total money in the bank, so catching him was pretty easy.
I've been audited a couple of times and the agent will interview your neighbors about your life style/entertaining/vacations etc. Get your bank records and even follow you the day before the audit. What good that does puzzles me, since the audit is for a few years back.
Bottom line is that its not worth the problems. God forbid that you tell your wife/girlfriend/mistress you will be dead meat sooner or later.