Very good points. 1,2,9,and 10 should get you started. Just don't wait fast results - unfortunately often today even big corporations see capacity planning just a bunch of programs and reports? Capacity planning is very interesting but almost a forgotten "art" currently.
Learn modeling or even simulation of systems and infrastructure, on log run there will be surprises and it is nice to know boundaries before they happen.
Don't assume anything - in theory an application or system may have certain characteristics, the reality may be totally different. Very important to know what business plans executive branch and for example security organization have, they don't always think capacity when making their decisions! And markets change all the time!
Don't try to make too long forecasts before you have enough(?) data - you will be hard pushed by other organizations and executives but try to explain why unknown is impossible to estimate. Watch patterns and trends both technical and business, they often tell more than just cold facts.
Some large vendors have very good resources, some don't - participate conferences, seminars, user groups, talk to other capacity people - unbelievable amount information is available but has to be found and filtered, especially vendors try to push their views and often have no idea of you business model.
From beginning make the data you collect compatible with future data, any numbers and figures must have the same meaning next year, otherwise they will come (almost) useless and may even give wrong results. Watch rules, regulations, laws, etc - they may have a huge effect sometimes and even more so if the company works globally. Don't forget disasters, backups, restores, etc.
And so on, remember every business environment / infrastructure / etc is different and ever changing so there are no canned answers in capacity planning. And be ready for political fights, those always happen when talking about resources and future!
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