Things to not do in your Salesforce Org - Do what I say, not what I do. 

• Not setting up Mini Page Layouts - they are so useful. 
• Not having Help Text on EVERY single custom field. Also include a description for WHY it was added. 
• Adding formula fields to child object so show data from the parent object - just add it to the mini page layout or Custom Report Types instead.
• Heaps of checkboxes - Unless they are used for frequency reporting in place of a Multi Select Picklist. I HATE checkboxes because they can't be an explicit NO. 
• Not turning on Enhanced Profile View
• Not investing in a tool like Rollup Helper or using Declarative Lookup Rollup Summary. There is so much that can be done with these tools. 
• Not installing apps like Kugamon Account Contact Sync 
• Not installing or setting up any form of dupe blocking. 
• Setting up reports to be exported - Question why you exporting data. Use Excel Power Query at least (unless over 2000 rows). 
• Export your data and back it up. 
• Not clearly identifying your test data so you can exclude it from reports. 
• Creating custom objects when a Standard object can be used. 
• Not linking with external apps - eg Mailchimp or Campaign Monitor
• Not using Do Not Call and Do Not Email fields
• Adding values into picklists that are used for only a short time
• Not setting up page layouts to be logical order of fields and groupings.
• Keeping related lists on page layouts that are not used. 
• Not removing Yahoo Maps custom links on Page Layouts - come on - you KNOW you will never click it - especially now with the new integrated maps
• Not setting up Record Types. 
• Not setting up a custom App with only the tabs you need.  
• Adding system only or reporting only fields to page layouts - just use Salesforce Inspector Browser Extensions
• Using Activities instead of the Feed. 
• Not using the Feed.
• Adding Status fields on the object. 
• Worse still, adding Latest contact fields on the object. USE THE FEED!
• Adding ALL the information into the Description field. 
• Having a text field rather than a picklist when reporting on that field and expecting consistent values. 
• Reporting on Tasks - WHY - who ever looks at these reports? Do you not trust your staff? 
• Using Tasks to remind people to do stuff. See Action Views, Tasks and Report Alerts
• Having Excel spreadsheets that relate to any data in Salesforce
• Doing an import into multiple objects on a regular basis and not using something like Skyvva or one of the Excel based tools to do it quickly and easily. 
• Having ANY fields in your org that you don't know what they do. 
• Having ANY records in Salesforce that you can't immediately say why they exist and what benefit they have to the business.
• Importing data and not including a Data Source - where did this data come from? 
• Using the Account or Contact record as the place to update information about other records - eg the status of outstanding payments - unless of course it is automated by a Rollup. 
• Entering Names in a text field - eg the Accounts Payable name - NOPE. Add a Contact with a Type of Accounts Payable, and add that as a lookup to the Account. 
• Ammending the ALL views to have filter criteria on them so they don't actually return ALL records. 

Reports

• Not hiding unused report types
• Not deleting sample reports