You have to use a real credit card, as many measures have been put into place to detect pre-paid cards.
Also, you can't complete a given offer twice using the same card, even if you switch IPs.
It's important to read the cancellation terms & conditions of the offer before filling out any offer. If you fill out an offer, and then immediately cancel, even after going green, the person you worked with on FLR may not get credit and will ask that you refund them. Or if you go directly with the merchant, you may not get paid by them either.
Cancel *only* at the last minute (such as a day or two before the trial ends).