I highly recommend Cluster Counting. It's quick and easy to learn and use, the math is simple, and, maybe most important, it takes just a couple of minutes to "get back into shape" after a long layoff.
The first seven clusters below are from Jack Kissane's Cluster Counting article. Those, and a few simple shortcuts explained in the article, are all you need. As I started using the system, I began to spot other clusters that are easy to count. Some are just variations on Jack's, but they come up so often I just added them to my "vocabulary."
 Center point of 5-prime x 10 = 60 pips |
 42 pips |
 70 pips |
 30 pips |
 40 pips |
 62 pips |
 40 pips |
 50 pips |
 50 pips |
 30 pips |
 74 pips |
 60 pips |
 57 pips |
 66 pips |
 20 pips |
 30 pips |
 60 pips |
 70 pips |
 100 pips |
 Sum of center points x 4 = 52 pips |