Only track KPIs that you are actually managing against. If someone asks you what you'll do about a certain number if it's not performing well, you need to be able to answer. If not, don't include the KPI.
Make sure the KPIs are very well defined, including a descriptive data source. They need to be easy to understand by everyone.
Automation & Ownership
Prevent manual data management. Ideally, every KPI has an automated data feed. If not, make sure to explicitly declare how the data gets generated and ensure consistency.
Every KPI needs a data owner. If automation fails, the owner should update the number at a set time each day or week
If you cannot keep the data for a KPI up-to-date, consider dropping it.
Stick with it.
As your marketing function evolves you will learn. It will be tempting to update the definition of your KPIs as your team grows up. Limit changes. An imperfect set of KPIs that stays consistent over time is better than KPIs that constantly change. Trying to make KPIs perfect while constantly losing historical data is a recipe for failure.
It will take a few months of discipline to get the most out of measuring and managing your marketing function based on data. Insist on a daily/weekly routine to look at, and talk about the numbers.
When the numbers are still small and trivial, it's the ideal time to make sure you're measuring the right things and course correct.
Benchmark - Numbers in isolation are meaningless.
Include actuals and original targets. Don't bother with forecasts or adjusted targets. Focus on closing the gap if you're behind, not on coming up with new (still arbitrary) goals.
Provide a trend over time, ideally as granular as possible. Even if daily or weekly data is not very accurate, it is good to capture this to potentially uncover patterns in the future when the numbers become larger. Also include YOY, QOQ, MOM, and WOW comparisons.
Track attributes to benchmark your KPIs by marketing channels, audiences, segments, and if applicable, competitors (for example for SERP positions).
Keep it simple
Use Absolute Numbers - Try to stay away from percentages. Use absolute numbers where possible. It provides the best insight into real growth and performance trends. For some KPIs like conversion rates % makes sense.
Don't overthink tools - Google Sheets or Excel are excellent dashboard tools. The conditional formatting in Microsoft Excel is great. Both can be shared as online web browser-based tools. You can invest in fancy BI tools or graphing solutions later. At this stage, it's all about getting and using the right data
For automation, a great tool is a simple calendar reminder for the people on your team who own each KPI. Just schedule time in everyone's calendar the same time every week or day where everyone updates the actuals of the KPIs they own.
It pays off to apply the above principles with iron discipline. I believe it can be the difference between success or failure for a young, fast-growing startup.