Steve Barnett makes some great points on long-term planning in Plans, Details, Dates, and The Future¹. I especially like the point about how user research fits into planning:

Before development starts on a new bit of work, you should be building prototypes and doing user testing with them. This always results in some changes to the plan, and often results in rather large charges. You can’t plan what these changes will be: you don’t know until you’ve done your user testing.