When devising a strategy to improve performance, you need to consider the workload in two environments: test, and production. Ideally you should have access to both, but often tuning must be done without the benefit of a test system.
Test Workload: In a test environment you can use strictly defined workloads to model how changes to performance parameters may affect your production system. Consider modeling your production system with a small subset of transactions from it. By running a wide variety of SQL statements from different applications you can create a rough sketch of your system. While it may not perform exactly the way the production system will, it can help you discover unexpected effects before they occur in production.
Production Workload: In contrast, you probably do not have a great deal of control over the size and nature of the workload in your production environment--you can measure it, looking for maximums, minimums, averages, and variances over time, but it is almost impossible to accurately predict exactly what it will be. Instead look for trends that can help you predict future capacity requirements. For example, will you need to buy hardware or invest in additional performance tuning to support a rapidly growing workload.