Control the amount of customfields is the system
Every new customfield affects the system performance so as a Jira administrator, you have to check if this new customfield is really neccessary. Moreover, you have to cleanup the customfields list from time to time.
Check the current list of available customfields before creating a new one
You could always reuse the customfield with configuring it's context or adjusting it's values.
Use universal names
This makes you able to reuse more. For example, use "Lenght" instead of "Hall length".
Don't forget to translate customfield names
It's confusing for users when smth in the system isn't translated to their language.
Customfield context. Repetitio est mater studiorum
Avoid using Global customfield context, this may affect your system performance. Use defined context for every customfield you have.
Default value - use it only when neccessary
Avoid using default values since Atlassian doesn't recommend to do so.
Don't use the same name for two customfields
Two customfields with the same name will confuse you while adminestireng the system or writing automation, and users while searching and looking through dashboards. Sames goes for names that are used in Jira by default such as Status, Resolution, etc.
Don't use space or scpecial characters in customfields names
Check the customfield name with online services like this to avoid spaces or special characters which will confuse users or make a mess in scripts.
When you don't need this customfield anymore
Think of issues that contain values in this customfiels. Are you ready to flush this data or you need to migrate it before deleting the customfield? Don't forget to add (deprecated) to the customfield name so nobody will use it in a new workflow. Delete the customfield only when you know for sure that this won't corrupt anything in Jira.
When you need A LOT of customfields
Don't forget you could avoid creating new customfields by changing their customer portal names only (if we speak about Jira Service Management). In additions, it worth to check the addons such as Extension for JSM which contains Bundled fields. Bundled field is one of the best ways to optimize the amount of custom fields.
Customfields' side-effects on system performance
Always remember that too many customfields may slow down your Jira. Chechk this Atlassian knowledge base article to know more.