One way to get money for your show is to just ask for it from your listeners. Simply putting a PayPal tip jar on your site and mentioning it in your show can generate revenue. Steve from Escape Pod was generating enough income from donations that not only was he able to purchase stories to be read on his show, he actually was cash-flow positive.
TWiT has taken the donation model one step further and offers up a forum with access only to those who donate to the show. With well over 100,000 listeners (some estimates put the audience at over 200,000), it is easy to see how a donation model could generate a significant amount of income for TWiT. So by offering up a members-only forum, TWiT is enticing its listeners to donate to the show.
If you are going to have a tip jar or a Donate Now button, your show should be commercial free. If you are going to subject your listeners to commercials, you should not expect them to also donate money to the show. That would be like Donald Trump coming on at the end of The Apprentice and asking you, the viewer, to send him more money.