Disclaimer: Do not try any of the methods below with the exception of #1 if you have verified and double checked that the conditions are met. All test are at the assumed voltage of 120 VAC.
There are several ways:
1- Find the breaker and turn it off is the first and best way.
2- Be completely electrically insulated from any other potential sources or conductive materials. (Also similar to a bird on a wire.)
3- Make sure that the conductor is properly insulated and hold the insulated portion of the conductor only in your hand. (Works with insulated tools holding the conductor also.)
4- Come to a complete and utter understanding that electrical current is going to pass through you. Ensure the 20 amp circuit is not GFCI, EPD, or AFCI protected (this method may not work if these are present). Then after applying the conductve solution used for connecting heart monitors to both your hands and bare feet, grab the 20 amp line conductor in your hands while having the neutral firmly wrapped around both feet (three or four coils ought to work). You will be electrocuted (a different magnitude than shocked though used incorrectly for shocked in many cases - it means death occurs) and having prepared yourself fully with the knowledge you should not be at all shocked that death occurred.
(Sorry Ryan, just my dry humor comming through.)
In methods 1-3 if there is no path for electricity to flow, it can't, therefore no shock.
The easiest way to think of it is to look at a bird on a primary wire. He is fine because there is no return path. But if you have ever seen a squirrel explode on a pole transformer you can see a small "grounding" error can have less successful results.