I'm hardly an accountant, CFA or politician, but the simplest thing is to define a taxation system (our progressive one is sufficient) and simply tax all every dollar that ultimately goes to a person at that rate. Finally, eliminate all deductions.

That should resolve most of the problems. The problems begin when we start saying certain things are virtuous - oh buying a home is a good thing, let's incentive that and allow people to deduct. Donating to charity is good - deduction! Investing in something in the long term is good - lower tax rate! Environment is good! Energy credit.

Enough. Who exactly benefits from all of these deductions? You guessed it! Regardless of how you feel about the actual deductions, if you want to eliminate "tax evasion" (I use quotes because it's not even a bad thing, it's perfectly legal) - that's the only way.