Most workforce planning advice is written for organisations with a planning function and survives contact with a real budget round about as well as you would expect. These six are the ones that hold in a team where planning is part of somebody's job, plus two pieces of common advice that do not and why.
One: plan hires, not growth. Two: measure your own attrition
Every plan understates hiring by the number of people who will leave, and the correction is arithmetic rather than judgement. Use your own leaver data over the last twelve months; a borrowed rate is about somebody else's workforce and it moves every downstream number in this list.
Three: write the approval deadline, not just the headcount. Four: count the ramp
A requisition approved too late cannot start in the year, so the date is the actionable part of the plan. And a plan that books hires as productive on their start date overstates capacity by the ramp, which across thirty hires is a real number rather than a rounding.
Five: keep assumptions with the plan. Six: review on evidence, not on a calendar
Six months later the question is what was assumed, not what the number was. And a plan reviewed quarterly regardless of whether anything changed trains everyone to ignore reviews; a plan reviewed when attrition or the target moves gets read.
The two that do not survive
Benchmarking your attrition against an industry figure, which produces a target nobody can act on, and modelling attrition risk on a few hundred people, where the sample cannot support the confidence the output implies. Both are common advice and both are worse than measuring your own numbers.
Questions people ask about workforce planning best practices
How detailed should a plan be?
Detailed enough to act: by role and by quarter is usually the right grain. Monthly by individual is a forecast of things nobody can predict.
Who should the plan be shared with?
Whoever will be asked to defend it, which is more people than usually see it. Circulating the assumptions is what prevents the private arguments.
What is the most common single error?
Confusing growth with hiring. It is arithmetic and it is in almost every plan that has not been checked for it.