I "invented" this putting drill at the course today. I like it because I feel it focuses on the 3 elements of putting: Lag Putting, Making Makable Putts and Pressure (at least that's the elements of putting that I think about). On a practice green, pull out 4 balls (or more if you want to make it harder) and find about a 40 foot lag put. The goal is to two putt all of the balls. If you don't two putt one, then find another 40ish foot lag putt. Putt to make each each ball before you move to the next lag. I like to keep moving around so I am practicing my feel for the greens and don't get comfortable with one put. Obviously this drill is a good practice for your lag putting since you'll be doing it a bunch. It's also a chance to make practice the mid-range makable putts since you'll attempt to make each putt. There will be times where you have a chance to redeem a bad lag and gain confidence. Lastly, this drill replicates pressure which is probably the hardest component to practice. Since any one missed put forces you to start the lags over. It may not sound like a big deal, but when you have a chance to end the drill you will be feeling the pressure.
I hope this is one that helps you on your way to better putting.