I've got a few strategies that work for me.
 
 1.  I plant every week, usually on Saturday.  I don't plant a lot, but I make sure that I plant a few things every week to assure that I'll have a continual harvest.  For example, I'll plant 2 or 3 (at the most) cabbage plants in pots.  I know that I'll never eat more than 2 cabbages a week, but I can give one away or make a jar of kraut if I've got too much.  When those cabbage are big enough to transplant, I'll drop them into a space.  I'll plant 6 to 8 sweet corn seeds directly into the ground once a week from March till late June, knowing that I'll want sweet corn throughout the summer.  Summer squash or zucchini -- MAYBE 1 plant every other week.  It takes a measure of discipline, but if you do it weekly, you aren't tempted to dump the whole package of seeds in your hand and go hog wild.
 
 2.  I can see what I've got growing in my nursery area, so I'll transplant only what I anticipate I'll use.  Sometimes I'll have the seed out there and I'll have a half-dozen pots filled with soil, and before I know it, I've planted six pots of jalapeno peppers or something.  If they all sprout and all grow, I realize, there is no way I need 6 jalapeno plants, so I may only transplant 2 or 3 to the garden.  I can then either 
compost the other ones or give them away.  The nursery/cold frames are a sort of shock absorber or buffer before it actually goes into the ground.
 
 3.  The chicken tractor only gets moved about once or twice a week during the heart of the growing season.  I'll let the girls scratch around and prepare a little garden bed directly beneath their chicken tractor.  The footprint is 4 feet by 8 feet -- the size of a sheet of plywood.  I'll move the tractor (again, usually on Saturday), and then I've got the perfect prepared garden bed to plant.  I'll always do a bit of spinach and lettuce, a few herbs like dill or basal, and a couple of other plants.  I'm not tempted to over plant because it's such a small space.  Every week, another 4 x 8 bed, all throughout spring, summer and fall.  In the winter, I run the tractor over the lawn more frequently.  We've always got salad greens and always have a bird-prepared bed to plant in.
 
 If you come over to see my garden/orchard, there are these odd rectangles of stuff growing: some greens, some onions, some squash, some corn or okra . . . 
 
 4.  I still plant too much, but because seed is free (we gather all our own) and because the birds always need something to eat, I don't see it as wasteful.  It allows us to be generous.  Can your really EVER plant too many watermelons?  (Well, yes you can, but folks will love you for it).
 
 5.  I remind myself that one of the principles of permaculture is abundance, and it's OK to return some of that abundance to the compost pile or chicken tractor.  Or I use it for social capital within the neighborhood.  Who doesn't like getting a big bag of freshly picked heirloom tomatoes?