Rather than accept say form encoded parameters it is usually more convenient (for the client and in the Apex code) to accept a JSON string that holds the parameters. For the POST case:
@HttpPost global static Result post() { String jsonString = RestContext.request.requestBody.toString(); // Use Apex JSON class to parse ... } For GET parameters you have to do your own string manipulation of the URL that you can obtain from the static field RestContext.request.requestURI. (Or better use params as Jitendra illustrates.)