Google Tag Manager best practices include planning before implementation, having a clear account/container structure and ownership, using proper naming conventions, giving GTM control only to the right people, leveraging workspaces, considering constant variables, using Lookup/Regex Tables, using Custom Templates if possible, implementing server-side tagging, consulting with developers before using unknown Javascript codes, utilizing the Data Layer, handling opposition from developers/CTO, asking developers to add data- prefix or IDs to important website elements, monitoring page speed performance, always testing before publishing, using debugging tools, checking real-time reports and debug views, writing clear descriptions and names of version releases, publishing/creating versions in smaller chunks, collecting data and verifying first, avoiding double tagging, following industry news, and checking online courses for GTM knowledge.

Lookup/Regex Tables in Google Tag Manager are used to manage multiple values in your tracking setup, simplifying the number of required tags. A lookup table variable takes another variable's value as input and sets an output variable based on that input. For example, you can use a lookup table to manage different Google Analytics Measurement IDs for a production and development website. Regex Tables work similarly but support regular expressions for more complex conditions.

It is important to consult with developers before using custom Javascript codes in GTM because such codes can potentially break website functionality. Even if the code comes from a well-known and respected source, it should be reviewed by a developer to ensure it does not introduce any issues or conflicts with the existing website code, which could lead to significant financial loss.

To handle opposition from developers or CTO against implementing Google Tag Manager, it's important to inform them about the features GTM offers that allow for control over what the marketing/analytics team can do without compromising security. For instance, GTM has allowlist/blocklist features to prevent the use of certain tags and policies for Custom Templates that can be configured by developers, offering a balance between control and flexibility.

Server-side tagging with Google Tag Manager offers more control over the data shared with third-party vendors and increases the accuracy of the data collected by the business. Introduced in 2020, server-side tagging moves much of the data processing from the client-side (browser) to the server-side, which can improve website performance, enhance data privacy, and reduce reliance on browser-based tracking. However, it may incur additional costs and is not suitable for all businesses.