1. Benchmark. Establish what IT is spending per employee and as a percentage of total revenue. Compare this to others in your industry. You may have to do some sleuthing, or even hire a consultant, but it's really not that hard to figure out what your benchmarks are, especially if you work at a public company, a not-for-profit, or a government agency, and you have any kind of a network of peers.
2. Make sure that IT really is respected. You say that IT is respected at your organization? Great. Prove it. Survey your direct customers. Find out what IT is doing wrong, fix that, and watch your numbers improve. Communicate about it. Rinse and repeat.
3. Get face-to-face. A mentor of mine once told me that the need for face-to-face communication proportionally increases with the complexity or the emotional content of the communication. So: email is worst, a phone call is better, a videoconference is better than that, and face-to-face is critical. Hint: If IT is not respected, expect that getting to a face-to-face meeting will be difficult. See also point #2.
4. Work with middle management. Busy execs are, well, busy. So expect that top management will (a) be tough to get a meeting with and (b) want to keep those meetings as short as possible. Sometimes it's just easier to work with middle managers, who can be highly influential with the top management team and who generally have some resourcing authority. My team has had great success doing a low-hanging-fruit project with middle management. It establishes quick wins and proves the value of better-resourced IT.
5. Be clear about what resourcing means. You must be very clear about what you are doing with the resources that you are currently allocated. We are usually willing to put money into a good investment, but when we do not know the results of our previous investments we are extremely leery of adding more to the pot. If you do not share IT results in a cogent, consistent way, you can expect that execs will be inconsistent about investment.
6. Make specific "asks." Don't just ask for "resourcing," ask for specific things. Nobody wants to take out a loan just to give someone else $300,000. But when prospective homeowners see a wonderful house in a wonderful neighborhood, they are more than willing to be on the hook for that loan. If you need staff, ask for staff, and say what that staff will do. If you need capital investment, specify what you need and what the business benefit will be.
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All of that said, there are situations where IT perception is good, IT is being proactive and showing results of investments, yet somehow it's still struggling with resourcing. There are situations where execs simply do not prioritize IT, in some cases because they do not fully understand (and in some cases they do not want to understand) what digital and technology can do for the organization.
In most cases, you want to get your house in order before complaining about management's lack of priority on tech. But if you have gotten your house in order? That's a difficult crossroad: It's likely that the CEO simply doesn't prioritize technology, and that you have some tough career choices in front of you.