Yes, I actually do a meal plan every week. It's like most plans, once the actual kitchen combat starts it is subject to constant revision, but I've found it indispensable for keeping (1) the fridge under control, (2) chaos from taking over when it comes to dinner time, and (3) monitoring the balance of our diet over the course of the week.
I don't plan breakfasts and lunches for the most part. Mika just takes leftovers for lunch, and kid breakfast and lunch are moving targets at all times. Since the kids usually get a plate of stuff at dinner that includes what we're eating, plus some other choices, Mika and I will have the conversation "what did Iz have for protein today? Did Duncan have a green vegetable? " etc. before deciding what to put out on the plates as extras in the event the main courses don't get eaten in their entirety. Not rocket science - the meal plan is just good discipline. When we make variances, I write them down, and then I can look at the plan -- I have to look at it, it's at eye level on the fridge every week -- and adjust the rest of the week, or the following week, accordingly.
Anyway, I keep copies of all our meal plans, and the little sticky notes are the really seriously underorganized part of my kitchen notes (recipes are in relatively good shape, emphasis on relative) and the part I am hoping to get into the blog for easier reference. So I expect this will be a weekly entry on Wednesday or Thursday every week. We normally shop on Thursdays, to correspond with when we get our vegetable box from our CSA (varies in the winter, since we use a different CSA), and the plan starts with the vegetables, factors in special events (evening meetings, parties, etc.), fills in pasta night (Mondays - sorry, Prince Spaghetti!) and one selection by Duncan (he likes to pick Saturday nights), then is rounded out for balance in starches and proteins, and finished off by any experiments I'm interested in or occasional special request from Mika (or urge from my own food id -- see the post on To Dive and Lie in Lasagne from some days ago). 
I keep this simple: one "main" course and two sides. I use my Grandmother's color wheel most days: at least one thing has to start life out green. One starch (sometimes none, particularly if we're having squash or something like that as our non-green vegetable). Quantities and proportions vary. If we're doing a One-Dish-Meal (ODM), I just sort of mentally divide the contents accordingly. I am not dogmatic about the courses, but the rule-of-three I think helps control portions (I was raised on the four-course meal). We put in red meat no more than once a week, and chicken usually about once a week. I have a processed food "mental coupon" - I "allow" one processed food item per week, and of late it's usually gone towards Duncan's pizza topping in alternate weeks (pepperoni).
One of the reasons i keep the meal plans stuck to the fridge for some weeks/months is it lets me go back through and see how I'm doing as far as balancing the diet, etc. I do keep a sticky note on the cabinet for ideas and suggestions. Between that and the CSA newsletter and random reading of recipes in the paper or occasionally on-line, we usually don't have a problem coming up with a 7-day plan.
Implicit in all this is the time allowed for cooking. I'm a realist by this point, despite my love of cooking, the boys can be pretty exhausting. So I try to keep simple/more complicated (in terms of complexity and steps, and cooking time) in balance, and also leave a simple meal for the middle of the week in the event Mika has to cook (if, for instance, as is the case this week I'm teaching one evening). I've also been trying to have at least one "cook ahead" meal a week to allow a little extra all-four-family time (or downtime for myself) after Mika gets home for the evening.
Spontaneity, you ask? Not always our friend. But I'll change on the fly if I see something I want to cook that night, if the day has gone badly and we need something that fits the bad mood of a child (or grown-up) or lower amount of time, etc. And often on Wednesdays, the last day of our food week, I will write in "Chef's Choice", which allows me to (a) do a kitchen sink recipe to get rid of whatever is leftover at the end of the week that simply will not keep, (b) factor forward or backward a meal due to some contingency (e.g. Mika has eaten up the leftovers I was planning on using, (c) the ever popular leftover night, or (d) Daddy's prerogative to declare a night off and get take out or go out to eat. The key to being able to be spontaneous is managing the fridge and the pantry (a topic for another day). One simply must keep ahead of the fresh food to avoid wastage, but at the same time have enough things on hand as staples to be able to do something interesting, if, say, we've invited somebody to stay and eat or whatever.
If this seems overly elaborate, it's not. It takes me maybe a half hour a week, and I'm making the grocery list at the same time, of course. I jealously guard that half hour, because it is a proven time saver over the course of the week. One problem we only rarely, if ever, have, that we used to have all the ime: looking at one another late in the day and asking "I dunno, what do you want to have for dinner?"
