Not my tip, but one that saved me recently. A six-month-old comment on a 9-year-old Reddit thread.

1. Compress the chair (all the way down), then turn it upside down. Let it sit upside down for 24 hours (the longer the better).
2. The next evening, leave it upside down and expand it (in other words, make it higher WHILE it is STILL upside down).
3. Then (without waiting another 24 hours) compress it (all the way down again), and leave it there for another 24 hours (again the longer the better).

Note that you will need a lot of force to get it back down in step 3.

Problem solved. (You may need to do this again in 2 years). No mess, no oil, no piston replacements or PVC spacers needed. Why does this work - you are forcing the oil back to the top (when its upright) again which allows the internal gasket to get coated with oil. The oil creates the air barrier that keeps the piston from shrinking as you sit on it.