Well, lets see... hmmmm. The most important thing its the legibility, and to have a good base to begin with the stamp. Search for a stamp sample, better if its a Photoshop psd archive, so you can modified as you well. The rest its easy, find a good pic, with not too many details.
Here's a good sample from Pakistanis [link]
For animated stamp, well, thats another history, but there are programs that easily can made those stamps.