Use case modeling is commonly applied to document requirements. Use case specifications (UCSs) are usually structured, textual documents complying with a certain template. However, because they remain essentially textual, ambiguities are inevitable. In this paper, we propose a new use case modeling approach, which is composed of a set of well-defined restriction rules and a template. The goal is to reduce ambiguity and facilitate automated analysis, though the later point is not addressed in this paper. We also report on a controlled experiment which evaluates the restriction rules and template in terms of their ease of application during use case modeling, and the quality of the analysis models derived from them by trained individuals. Results show that the restriction rules are overall easy to apply and that our use case modeling approach results in significant improvements regarding the correctness of derived class diagrams and the understandability of UCSs.