I've been evolving my Excel spreadsheet for the last 8 years.

I break the year into 13 4-week periods. Allows better comparisons because each period has 28 days and 4 weekends, unlike months.

In the primary sheet I track -

Date, Bike, Time, Miles, Avg. MPH (calculated), Cumulative Distance (by period), Cumulative Time (by period), Where I Rode, and Year to Date.

Organizing it this way allows me to have another sheet where I can compare months, mileage by bikes and a bunch of other stuff.

Pretty simple, little upkeep. I enter date, bike, time and miles after each ride, the rest takes care of itself.

I have also used bikejournal.com for the last 3 years.