Hey Liz, How's it goin'? Happy New Year!
When you say "we are doing man power planning...", who is included in "we"? Is it you and your team? Is it your manager's organization? Is it the entire HR organization? Is it the entire company?
Answering this question (who is "we"?) is significant because it shows the level at which the company is committed to dong the planning. The answer also makes it clear as to who you must convince (get the buy-in of).
Further, is this planning/budgeting only for HR organization or the entire company? The only way you can the job done here is by going the top-down route. Having a clear plan on how "they" (each of the organizational heads of various orgs in your company) can get this done would help.
Here are some steps (it is no different from selling any idea to the senior management and getting their commitments). You can cross out any/all steps you have already completed and consider the remaining.
1. Business case and cost/benefit analysis (BC/CBA) - It's a no-brainer, but still if you are asking this question, obviously it is not such a no-brainer to your senior management, after all. BC/CBA doesn't have to be fancy or elaborate. Just one slide containing why the company/organization needs to do the exercise of planning and budgeting, and another slide containing what it is going to cost them, and how it will benefit them (in terms of hard dollars, soft dollars, etc.)
2. Next (third) slide - Clearly articulate the information you are seeking from them (Current head count, future growth potential, required head count for next year (two years, 3 years, 4 years, 5 years) based on future growth potential, etc).
3. Next (fourth) slide - Clearly articulate a plan of action (for them), what do they need to do to get you such information (request each of your people-managers to provide staffing projections based on these set of questions, etc.).
4. Next (fifth) slide - What are the next steps? What are the timelines (for them to get back to you)? What do you intend to do with the information they provide you (Based on the information you provide us, we would keep reviewing it with you periodically and provide the right level of staffing so that you can meet your organizational goals, etc.). This information may also be given in your business case slide.
5. Next (Sixth) Slide - Questions and Answers (Open Forum) - This would be a blank slide where you would note their questions and the answers you have provided.
6. Final (seventh) slide - Commitment: List the names of the folks attending the meeting and seek their commitments on getting the action plan executed.
That's it. Then, take this slide deck and go to your manager (review and convince your manager). Then, along with your manager, go higher in your organization and all the way to the top (of the company) where there is only one person. You convince this one person right on the top, that person will ensure everyone else in the company is convinced.
There are a few things you need though.
1. A well-articulated powerfully presented business case (the business case is already strong. It needs to be presented in a powerful language as well).
2. Your will and attitude to convince people. Based on my previous interactions with you, I believe you already have the right attitude and will.
Hope this helps.
All the best. Do let me know how it turns out.
Best regards,
-Som G