The easiest way of course is to use dropdownlists.
Alternatives are to use the user-parameters as input for internal parameters.
F.e. you let the user enter the parameters you then define an (or several)
internal parameter that is/are the result of a validation function (you can
even use custum code to do the validation). The function does the parameter
validation and returns the result to the internal parameter that you use in
your report.

[quoted text, click to view]