Tom Mochal provides some examples of where you should plan ahead in a project.
Your risk management process allows you to evaluate and respond to high-level project risks. Some of these risks involve project resources and require that you consider ahead of time how you will respond if you need to replace or add resources. In fact, in some cases, you must actually plan ahead to understand what the contingency resources look like and how you'll get them if they're needed. This contingency planning could affect either labor or non-labor resources. Here are some examples of where you should plan ahead.
On many projects, if you find that work is taking longer than you anticipated, you might have the flexibility to ask for additional time and budget. However, if the deadline date is critical and can't be moved, you may not have time to look for new resources to get back on schedule. Likewise, if a member of your team leaves, you may need to find a replacement in very short order.
When deadlines are firm and the project deliverables are critical, you need to have some plans in place for finding resources when needed. For example, let's look at the YR2K projects of a few years ago. If you were entering the final six months of 1999 and needed more resources, you would not have time to spend three months finding people. You should already have had a plan for acquiring resources on short notice. This may have meant having employees or contract people in reserve to allow you to make staffing changes quickly.
A high incremental cost of resources. You may have resources that are less expensive when purchased in bulk, but very expensive when purchased incrementally. For instance, if the solution you are building requires new hardware, you may find that the price per unit is less as you purchase more units. Let’s say that you estimate you will need 100 routers, plus or minus 10. Your hardware vendor may give you a very attractive price for buying in bulk—perhaps 50 or 60% of the unit price. In this case, you may choose to purchase the full 110 now and have ten units in reserve. You would do this because the price to purchase the extra ten units now (as a part of the bulk order) is much less expensive that having to purchase ten units later, when the incremental cost would be much higher.
Long lead times
Long lead times for specialty resources. Sometimes there's a long lead-time to acquire hard-to-find specialty resources. If the need is critical enough, you may need to know ahead of time how to find these people on short notice if needed. For example, you may need experts in some obscure tool. One way to plan ahead is to work with the vendor to have resources identified that can be made available to you in an emergency. If an expert on your team quits, you will not be stuck. You would have already worked out a deal to have substitute resources available on short notice—even if only short-term.
You can see that not all projects require this sort of advanced planning. However, on some projects it's absolutely critical. The project manager should understand ahead of time whether there are resource risks such as those described above. In those cases, once you discover that you need these resources it may be too late to find them and still complete the project on time and within budget. So, do what a smart project manager would do—plan ahead of time and understand where you will go to acquire these critical resources when needed.