By guerilla testing I mean brief, informal testing performed on random people who happen to be about. Here, I'm specifically talking about testing on strangers in public place, not people within your organisation or family and friends.
Use giftcards for an establishment close to your testing area. Once for guerrilla testing, I set up a laptop just outside of a Starbucks near our office for testing. We had a lot of people agree to do the test for a $5 giftcard, I'm assuming, because they could use it right there and then. The test was very brief (8 tasks) that took about 10 minutes to complete. We had ten giftcards and they were all gone in about two hours.
Not sure the booze theory is scientific enough to follow those words of wisdom. Although I've tested in bars before the user has become inebriated. I tend to pick a place where people wait - train stations is a good example. There's always a cafe near by to set up and buy your patron a coffee.
Another thing I do, and it sounds pretty sketchy, is I will hunt down my patron and find an opportunity to small talk before introducing my testing.
For example. (I'll use the train station example again) Note the people staring at the timing boards and ask if they can't find their train. Help them out, confirm they have time to burn (because their train isn't due for 30 minutes) and offer to pass the time by taking a test while having a complimentary starbucks.
It means I don't have to approach people cold, I identify if they are free and give them little excuse to decline. It has allowed me to get a 50% hit rate on people accepting while before it was more like 1 in 10 or worse.