also ask for permission in the summer time, don't wait until the last minute. In the fall of the year the farmers are busy getting their crops up and can't be bothered by someone that wants to hunt. in the late summer crops are in and he can take the time to talk to you. also a cold coke doesn't hurt either.
like daveh says, leave the camies at home. it has worked for me. also if you get turned down ask if they know anyone who would allow bowhunting. i have found if you tell them you are a bowhunter you will get permission over a gun hunter."just my 2 cents"