According to Treasury Regulations, there are a few conditions every disclaimer must follow in order to be considered valid. First, it has to be delivered in writing to the proper estate representative within nine months of the will taking effect.
Also, you can't disclaim just part of a benefit or use any of it before disclaiming -- even just putting the check in your personal account can mean the difference between a qualified disclaimer and a meaningless document.
And finally, you're agreeing to be totally hands-off, no matter what happens next: For the purposes of that bequest, it's like you don't exist. That means you need to know who the next person in line is before you disclaim the benefit in order to keep from creating even more problems.