Probably the number one tip anyone could give you for improving your spelling would be to read a lot. We learn the spoken language by hearing other people speak and we learn written language by reading what others have written. Spelling is about how the word looks on a page rather than about how it sounds. The more you read the more you will recognize the correct format for a wider and wider array of words.
If you are not sure about how to spell a word correctly, look it up in a good dictionary. It doesn't matter if you have to look the same word up 10 times. Eventually it will stick and you will be able to spell it correctly without assistance.
Use your spell checker, this is really the same as above but much quicker and in a lot of ways more effective. As you type and the misspelled word is highlighted, right click and look at the options you are presented with. Work out which is the correct word and then compare it with what you typed to see where you went wrong.
Learn the common rules of spelling, there are many websites out there that can help with this such as Spelling City or English Grammar Software - Rules to Improve Your Spelling.
Learn about commonly miss-spelled words. Words like they're/there/their, its/it's and to/too/two come to mind. Also make a list of words you commonly miss-spell, if you see that you miss-spell the same word over and over write it down somewhere, then find the correct spelling and write that next to your miss-spelling. You can come back to this list and focus on learning these words until you know them off by hart.
American vs British spelling. In the good old US of A we spell many words differently to the British. For instance nite as opposed to the British night. You need to make sure any resources you use (books, websites etc.) are appropriate for the version of English you want to learn
