There's a lot of history and tradition and law surrounding the heraldry of knighthood. Fortunately, we can pick and choose which of those rules and traditions we follow for LARP characters. I've always been a bit interested in heraldry, as a fan of knights and of history, so I've read quite a but of info on the subject, but here's the basics.
1) Coats of Arms were issued for individuals, not families. A son might take up his father's arms, but there are no familial shields. However, that's a rule LARPers could choose to ignore.
2) The main part of the arms is the shield. Your creation for your LARP character could be mirrored on his shield. Shapes of shields, style of mantle, and style of helmet on the crest are all determined by the artist or herald, so feel free to experiment with these. You may, for instance want your shield to be the same shape as your character's shield. If this is a crest for a modern-day character, perhaps a vampire who has been around for centuries, you may want the helm and shield to reflect a certain time period or location.
3) Heraldic symbols use colors and metals (silver, often represented by white, and gold, often represented by yellow). In general, metals should appear on top of colors and colors should appear on toil of metals or other colors.
I found a site with lots of info on heraldry. Of course, they're trying to sell you a custom designed Coat of Arms, but you're creative enough to take their tips (and maybe a couple of their images) and make your own, like I did. http://www.fleurdelis.com