There are several resources one can use for studying for the amateur radio license examinations; here are the ones we personally recommend:
Studying for the extra-class exam is harder; one can buy a study guide from ARRL or from the previous links.
Not only is studying recommended, but so is testing. Practice exams can be found in many forms, including applications for Android or iPhone phones.
The one we recommend most is QRZ.com. Not only is it useful for practice exams, but also for looking up call signs, confirming contacts, and has many useful links to other amateur radio sites.
More information can also be found on the ARRL website.