Some good ways to improve spelling are reading books, keeping a spelling journal and playing word games such as Scrabble. Beyond that, just regular practice writing words can be very helpful.
One unique tip is to write words that you have trouble with on sticky notes, then place them on your mirror. This way, every time you look in your mirror, you'll also be working towards memorizing a word that you haven't learned to spell yet. Spelling programs such as Ultimate Spelling can also be a helpful tool. The programs include practice and games to help you learn lists of words that are difficult for you.