You have a brilliant idea and you want some feedback on it.
Here is a step-to-step plan how to proceed :
1. Go to your local pub and tell everyone that you are looking for some smart guys. Skip the first volunteer. He is drunk and useless for playtesting purposes.
2. Offer all your victims a drink. It is important that they start the test with a positive attitude.
3. Important : do not begin explaining the rules of the game right now. The folks are not paying attention anyway, they can't memorize all the details and they don't care a sh**t. Instead just start playing the game and explain what players must do when it is their turn to play.
4. During the game, interfere as much as possible. Give suggestions how to play, give strategical hints and explain why you love your own game so much.
5. Do not to forget telling some good jokes in order to create a happy feeling and relieve the tension (if someone gets upset from your ticks).
6. When you are hobbling behind, do not hesitate to suddenly change the rules in your favour. However, do it in a clever way. Tell the other players that you are just reminding an important rule. If they start complaining, just tell them that you are not playing, but playtesting a game and that changes are very common during a testing session.
7. When you have won the game (which should be evident, as you alone know the dirty little tricks how to win), offer everyone a new drink and ask in general how they have liked your concept. However, now you must be very talented. If they want to express a certain reservation, it is important to explain why the playtest has been a little outbalanced and is not representative : they had bad luck with the die rolls, they did not follow your advise in turn 1 or they simply do not have understood the key elements of the game. Do not tell them that you have expected to play with some smarter guys and that you are the one who is disappointed most. At this moment, you must use all your diplomatic skills.
8. Don't take notes. As you seldsom gets a usefull suggestion, it is not worthwile to spoil ink and time writing down some silly comments. Remember that you are the designer, not them.
9. Next, thank everybody for the nice evening and express the hope that they will buy your game once it will be published, which will occur soon. Do not tell that you will never ever want to play with those losers again. They just participated at your playtest for the free drink.
10. Finally, go the girl at the bar who was smiling at you during the whole evening and ask her : "Do you like playing games?". She might answer : "Depends which game...". Now, you can reply : "Follow me, sweetheart". When you hear the tickling of her high heals behind you, you can say to yourself "mission accomplished!".
Conclusion : for a successful playtest, check first if there are some nice girls at the bar, waiting upon a handsome man with brilliant ideas. If not, go to another pub.