In addition to following the writing guidelines, you should try to omit the following mistakes. (Of course, this is not an exhaustive list. I've only written up some common problems I've seen in the past. So, try to avoid mistakes in general. =)

GENERAL:

• do not follow instructions
• a section that only has a picture
• a section that only has a table
• lots of irrelevant information that is not part of the argument
• errors in previous assignments were not corrected but the material was handed in again in upcoming assignments
• suggestions are not well justified
• no introduction
• no conclusion or summary
• colloquial use of language
• typos and grammatical mistakes
• errors in previous assignment not corrected, thus, produces compounded errors in future assignments

TRENDS:

• the marks generally are quite high for A1, goes down for A2, and goes way down for A3
• the number of pages in the submitted assignments unnecessarily doubles from A1 to A2, and quadruples from A1 to A3

FEASIBILITY STUDY (Assignment 1):

• numbers (e.g. salaries) are not supported by evidence
• the table of comparison of alternatives is hard to read

REQUIREMENTS ANALYSIS (Assignment 2):

• confusion between functional and non-functional requirements
• functional requirements are too high-level
• non-functional requirements are too high-level
• don't understand the difference between requirements analysis (the system and the external world) vs. design (the system internally)

UML (Assignments 2,3):

• confusion between ER diagrams and UML diagrams
• class diagram not included!
• incorrect syntax in drawing UML diagrams
• inconsistencies across different diagrams
• confusion between the idea of an actor and the idea of an object
• confusion between the idea of an object and the idea of an instance of an object
• diagrams have no accompanying description

DESIGN (Assignment 3):

• lack details in hardware/network specifications
• does not go back to ensure the reader that the overall design meets all the requirements
• don't understand the difference between requirements analysis (the system and the external world) vs. design (the system internally)

APPENDICES (Assignments 1,2,3):

• lack of details for the interviews
• not referenced in the main body of the report
• not ordered according to the references in the report
• no description or discussion