One of the toughest parts of playtesting is simply finding testers. Not every group sees “let’s play this broken, not-fun-yet game” as a strong pitch. 😉
It can help a lot to take your game to an event expressly meant for testing. Everyone there knows what they’re in for, and is (presumably, hopefully) in the right frame of mind to give useful criticism after playing a game that’s not quite ready for prime time. Rather than having to convince people to give up some of their entertainment time, you’ll be able to work with people who want to be playtesting.
Luckily, it’s generally not hard to find playtest events. I’ve never encountered a local game group that wouldn’t take time out at least occasionally to try a regular’s new design. If you’re in the New York area, the NYU Game Center has Playtest Thursday at 5:00 every Thursday–it’s open to the public. The Unpublished Games Network puts on Unpub events.
Give these, or other similar events, a try. I’ve found Playtest Thursday incredibly useful; I’m sure that it, or like events, will be valuable for you as well.