This is a difficult one to answer. First, build a strong business plan and convince management that the investment in IDS is well worth it. If your company relies quite heavily on e-commerce, ask them how much it will cost them if the Web site is down for an hour. The IDS then becomes an insurance policy against future probes, intrusions, or attacks. All IDS systems offer the option to set a site policy, which allows you to reset a connection that is determined in violation of the company's current security policies.
Some people hack their own systems from the outside to prove how easily it can be done.
However, this kind of demonstration can backfire since it proves to management that you know where the system's weaknesses are, and that the problems could most likely be fixed with the proper configuration or patches. There's a new mindset that you're demonstrating a useful degree of readiness because you can detect an attack. In theory, the IDS alerts you and you increase your vigilance for a period of time, increasing your chances of detecting and countering a real attack. These days, audit tools are driving the market to give a picture in time of the status of a network. If your network is audited regularly by security analysts, you'll be criticized if their testing passes undetected.