The DIALOGIC system for syntactic analysis and semantic translation has been under development for over ten years, and during that time it has been used in a number of domains in both database interface and message-processing applications. In addition, it has been tested on a number of sentences of linguistic interest. Built into the system are facilities for ranking parses according to syntactic and selectional considerations, and over the years, as various kinds of ambiguity have become apparent, heuristics have been devised for choosing the preferred parses. Our aims in this paper is first to present a compendium of many of these heuristics and secondly to propose two principles that seem to underlie the heuristics. The first aim will be useful to researchers engaged in building grammars of similarly broad coverage. The second is of psychological interest and may be a guide for estimating parse preferences for newly discovered ambiguities for which we lack the experience to decide among on a more empirical basis.