I'm known to rarely offer to chop, but will accept what I consider a reasonable chop. A lot of factors go in that evaluation (Kyle has a good list). I generally like BG's idea of a chop based something on chip stacks. Once I was chip leader and a chop was offered that would have left the last one with less than the payout they'd receive for playing it out and losing. I didn't think that was fair, so I insisted on an equal chop. It seems crazy and I wouldn't normally have done it, but they wouldn't consider paying her the last place amount without it. She wouldn't have disagreed with the chop out of courtesy to the host and others.
One way that will usually work is that the smallest stack takes at least last place $ (however many places are chopping) plus a little more. If they don't get better than last place money, they have no reason to chop since they won't come out worse by playing. Then the other places take proportionate to chip stacks or something similar, but that only works if one guy doesn't have most of the chips. Usually 1st will get less than if winning and ideally everyone gets at least some more than their current place would pay. Without those basic factors, chops won't work.
Last week we got down to me and another guy who also almost never offers a chop, and he is far less likely to chop than I am. I'll usually go along if I think it's reasonable. Payouts were $136 and $91. Our chip stacks were pretty close -- he had me by maybe 10BB and both had over 30BB. He suggested each get $100 and play for the rest. I agreed, but regretted it because I then got 2 strong starting hands in a row. Went all in first hand, but his hand was stronger. However, I caught help on the turn and he didn't. Next hand I had a better hand, called his all in, and once again, he had the better starting stack. I caught help on the flop and he didn't. I joked that it took me only 2 minutes to blow $9. Of course, it could just as easily have gone the other way. I don't feel bad at all and was happy with the $127. I got lucky.
At that point, some type of chop made sense because he and I have similar track records. It was likely to get down to who got lucky. A chop benefits both. I'd have even taken more each and played for less. You never really know what is going to happen at that point in the game.
I'm more likely to chop in a "friendly" game because it is friendly. Some have said they are less likely. I'm not likely to be the one to suggest a chop, so if they aren't inclined, I'm always willing to fight it out.