Get a coach to practice. If you're just practicing for your school spelling bee this may not be necessary, but if you move up to district or state then a spelling be coach will be ideal.
Get a list of all of the championship words for the last twenty years. These are the winning words for the national spelling bee. While they won't necessarily be championship words again they could definitely appear as one of the words in the early rounds of the national spelling bee, so be sure to practice them.
Download the Consolidated Word List from the Scripps National Spelling Bee homepage. It will give you a list of words to learn from the 1970's through to 2004.
Learn word origins. If you don't initially know how to spell a word knowing it's origin, Greek, Latin or other roots will help you figure the spelling out. For word origins use Webster's "A Dictionary of Prefixes, Suffixes and Combing Forms."
Download "Spell It" from the Scripps National Spelling Bee website. It will give you a study guide to follow to best prepare for a spelling bee.
Follow Carolyn Corners 36 week spelling bee practice study course. Her course follows the standard school year and offers the best online spelling bee course available for free.