There are probably as many ways to plan your homeschooling year as there are to do it. And which is more important, really? For me, however, the plan really helps me.
Some other homeschoolers who blog have much more elaborate systems than mine, and I’ve linked to a few of them here. Obviously if you buy a packaged curriculum, you have their lesson plans already made for you.
Charlotte Mason recommends alternating subjects by type. History (in which we are absorbing information) can be put between a nature walk (physical activity + close observation) and math (which requires calculation). But wouldn’t want to do copywork and drawing back to back, because they both require the same type of mental effort.
We do most subjects (Bible, math, writing, reading) daily. We do foreign language and science usually three days per week. Once a week we’re at a homeschool enrichment program. I schedule picture study, composer study, and nature study once a week on different days. I write this down somewhere as my template, but it’s not set in stone.
Then I look at each of our content subjects (upper level math, history, science, foreign language) to get a rough idea how many lessons there are for the year. I make sure I have roughly a corresponding # of days on that subject. I pencil these into my calendar. For subjects which are continuous– workbooks, piano lessons, copywork, Bible reading– I don’t bother to plan. As long as we do a little every day, I figure we’re doing all right.
Each Sunday night, I pencil into my planner what we’re doing for the week. Is it our week to volunteer, or do we have guests coming? I plan less, accordingly. I write our plans, and then check them off when we do them. See:

my planner
If we don’t get to something, I cross it out (or erase it) and rewrite it on another day. As we go along, I tally the days we’ve “schooled” to make sure we’ve satisfied my state’s requirement for “days educating” (though we all know a lot of education happens on days we’re not formally “schooling”).
Other options abound. One of my friends writes out each day’s lessons (without a date) in a spiral notebook and writes the date at the top when they get to that day. The online planner at Simply Charlotte Mason.com does a similar things but then lets you designate which child it’s for, what the scheduled and completed dates are. It stores old lessons and assignments so you can generate a new combination of lessons for another child without reinventing the wheel. It looks very thorough and satisfying if you’re computer-savvy. Kendra at Preschoolers and Peace.com has great resources for planning and record-keeping. Several of my friends (who are spread-sheet lovers) swear by Managers of their Homes, which tackles the planning of much more than home education.