Good day, these points are about planning in general but I think they're particularly applicable to planning software projects. I'd be interested to read what you think about them:

1) Long term plans are generally more important while short term plans are usually more realistic. On the one hand, long term plans cover a larger proportion of your life and have room for more ambitious goals. They're also important for guiding and informing short term plans, making sure the small steps you take are in the right direction. Shorter term plans, on the other hand, allow less time for circumstances to change, less time for you to change your mind, or forget details of the plan. Without good short term plans, long term plans don't stand much of a chance of being implemented. Without good long term plans, your short term plans won't have as much meaning or purpose. It's therefore dangerous to neglect either scope for long.

2) However good a plan may be, it starts becoming obsolete as soon as you put it into action. Some parts will have different results, or take different amounts of time than intended. Obstacles will arise that you didn't anticipate, and your priorities may change. You will gradually forget some details of the plan, and you'll learn new things that might be used to improve it. Eventually the original plan may be impossible, and so it becomes merely a work of historical fantasy. For this reason it's vital to revisit your plans regularly. On the one hand you need to remind yourself of some of the details you've forgotten, and on the other hand you need to adapt your plan to the changed circumstances, and refine your approach in light of what you've learned.

3) How long you spend thinking about a plan in one go, is just as important as how long you spend in total. This is because thinking clearly requires uninterrupted time. Firstly it takes time to recall your various goals and circumstantial constraints. Then it takes time to think up options for what to do and consider the merits of each one. As this process continues, the ideas you cover become part of your short term memory. This makes them easier to recall next time they're relevant. If you're distracted from this train of thought, some of the ideas will start fading from your mind. When the distraction ends, and you return to the subject at hand, it will take time to recall the details of what you were thinking. The longer the distraction, the longer you'll need to get your bearings again. So it helps to concentrate for a least a few minutes in a row, if you want to think clearly and deeply about a subject. On the other hand, if you persist for more than two hours, no matter how well you concentrate, your natural attention limit will be reached and it becomes gradually harder to keep thinking about the same subject. Once you start approaching this limit, a distraction may prove useful. It will rest and refresh your mind, or at least those faculties you'd been exercising.

4) Making a plan more specific makes it easier for you to remember the right steps at the right time, and this makes the plan easier to apply. Having a more specific plan also makes it easier to measure the results by comparing them to what you'd expected. This helps you judge how far reality has diverged from your intentions and therefore how badly your plan needs revising. To this end it helps to decide not only what steps to take and the order in which to take them, but also to decide how long each task or component will take, when it will start, and what counts as finished.

5) Putting a plan into complete sentences helps you to concentrate and be specific. Your explanation doesn't need to be written or spoken; just coming up with it is enough to focus the mind and encourage precision. If you need further help concentrating then writing the sentences down will take up more attention than just thinking them up, and therefore leave less room for distraction. Writing is generally slower than just thinking, and is not practical in some cases, but it does make it easier to concentrate.

6) Even if a particular plan isn't very important, your approach to making it might be. Trying to plan well even when you don't have to is good practice. It helps you learn the skills, knowledge, and habits required to make good plans, so when it is important, you'll be prepared.

7) Some activities don't require much attention and would not suffer greatly if you made plans at the same time: Mindless or menial tasks like tidying up, having breakfast, showering and dressing, walking somewhere in easy conditions, having a rest, or anything else that doesn't require much attention, and won't get you into trouble if your attention strays from it every now and then. Taking advantage of these opportunities means having extra time in which to plan, without having to sacrifice the other things you wanted to do.

8) Another way to find time for planning, without sacrificing other tasks, is to find those occasions where planning will save you time. If a plan keeps you focussed on your tasks, helps you anticipate and avoid problems more effectively, and makes it easier to remember what to do next, then it might save you more time than it took to create.

9) Before a plan yields any practical results, it alters your expectations and general outlook. These things affect - sometimes greatly - how you feel about your situation. A good plan therefore starts paying off immediately, even before you finish making it.

(from http://notesonplanning.com)