Spelling programs are easy.
At least, that's what I've found. The elementary grades offer much opportunity to teach spelling, and basic word vocabulary. There are innumerable choices for curriculum.
But then, kids grow up. They can spell, and they can look words up in the dictionary if they don't know their meanings. This can, of course, get them "through". But when you begin reading more complex works, it can be tiresome to look up every word that you aren't familiar with in a book. (And...gasp....sometimes, kids don't even bother to look them up!) Even worse, they don't want to read the classics - they may feel intimidated by them.
So what do you do? Expand their vocabulary, of course! But you can't teach them every word in the dictionary - surely we all know this, right??? You can, however, give them a great big head start - teach them the Latin and Greek roots from which many of our words in English derive.
Once they know the "root" - their comprehension of a new, and unfamiliar, word becomes much easier to learn.
The best program I've found for this is "Vocabulary from Classical Roots" - by Norma Fifer and Nancy Flowers. A straight-forward, and easy to use program that we use starting in the 7th grade, and continuing through the 1st half of 9th grade. We cover 2 books (minimum) per year - A & B in 7th grade; C & D in 8th; E in the first half of 9th. I like the learning schedule advised in "The Well Trained Mind" (Susan Wise Bauer), and so do my kids.
Do look into a vocabulary program, if your children are past the age of "memorizing" spelling - the relatively small time-investment will more than pay for itself in the quality of work that your child will produce in his/her written compositions and reports!
Language is what brings our world together - the written and spoken word is so very powerful!
And remember this: the pen IS mightier than the sword.