So far, I think swagbucks has been the easiest, quickest and the most reliable way of getting Amazon GCs. I have got $25 in the past 2 months without any references !!
Mturk has small tasks to complete called HIT (Human Intelligence Task). Accept a HIT/task if you like the description and the money. Upon completion of the task, hit submit. Money will be transferred to your Mturk account when your task is approved.
Once you collect $1 or more in your mturk account, you can transfer them to either your amazon account and use it just like GC credit or to your bank account.
I like this specially because I get money faster unlike these survey sites where I need to wait for several months to meet their minimum points to encash it. And they take so long to mail the check!!