Spelling for Success
Spelling and reading abilities are correlated. Spelling success leads to reading success. In the days of the one room schoolhouse, students were taught to spell before they could read and were not allowed to read words they had not yet learned to spell. (See this 1851 article.)
Spelling well is every bit as important today. Learning phonics principles can improve your spelling. There are simple phonics principles that are also spelling principles. Learning these principles can improve both reading and spelling abilities. You can learn all of these phonics principles in the online spelling lessons. The majority of these principles can be found in written format here.
A good spelling book can also help. Most spelling books are just compilations of words that are often misspelled, thrown together with no organization. "Spelling Made Simple" by Stephen V. Ross, however, organizes like words together and also includes useful spelling rules. If you are looking for a basic spelling book that reinforces phonics teaching, Practical Spelling by Learning Express is a good choice.
Here are some more good choices:
How to Teach Any Child to Spell by Gayle Graham
Tricks of the Trade by Gayle Graham
Spelling Plus by Susan Anthony
The American Spelling Book by Noah Webster
Dr. Ernest Christman's "Rx for Reading: Teach Them Phonics" is aimed at the remedial reading student, however, it is also a good book for improving spelling.
The Sounds of English
Synthetic Series of spelling and reading by Rebecca S. Pollard. This is an excellent series that is free online from Google Books. It makes use of lovely songs and pictures, and is a delightful, well designed method. It will take a bit of time to get used to the method, but is well worth the effort. Pollard herself in her Synthetic Speller states, "Observe this order of presenting the work
1st. Blackboard drill.
Or, you can start with her First Book for Little Folks. "By combining in one book all of the work for the first year, it is believed that the author's plan of instruction is made more apparent to the teacher, and that the work is put in more convenient form for the pupil.” The First Book for Little Folks includes references to page numbers in Pollard's Manual for each lesson.
Pollard’s Complete Manual: Synthetic Method of Reading and Spelling by Rebecca S. Pollard
Finally, a warning against invented spelling: it is a bad idea. See here for a complete discussion of invented spelling. Practice makes perfect...and if you are practicing spelling something incorrectly, you are only reinforcing the incorrect spelling in your brain. Students need to learn to spell correctly, and they need their spelling mistakes corrected so they can learn the correct spellings of words.
Here is a good list of spelling rules.
Once you’ve exhausted the sound spelling correspondences, there is another level of spelling: spelling based on etymology and morphology. A root based vocabulary book will help you learn this level of spelling.
Finally, the best way to improve your spelling while also improving someone's life is to find a person in need of reading help and teach them to read with phonics. You will both be blessed!