limits of magic

Mistborn by Brandon Sanderson is absolutely amazing in its magical limits and rules which are very clearly defined fir the reader and religiously adhered to thus far. Gives me a great example of boundaries, rules and limits.

I'm trying to find a way to create such limits and rules in my own book. Eragon was similar as well. While that magic dis not require a reagant as in Mistborn, the user was limited by his physical capacity to use the magic he possesed. Both are very good examples.