One of the things I think you should do along with everything stated above is prove that you care. Think of it from a parents perspective, if you ask for a guinea pig for a day and go crazy about wanting it and then forget about it for a while then go crazy they will think it's just a phase, and that you will get bored, but if you take the time to calmly and slowly ask for it and explain and stick with it eventually they will realize that it isn't a phase because you didn't give up on it and that you aren't going to get bored of a piggie, well, that's what worked for me anyway. Good Luck!