Originally Posted by rekmeyata

I'm sorry I can't find your post on this, but is the seat post aluminum stuck inside a steel frame? If so soda will work but then you have to rinse it out with clear water afterwards to get the sticky sugar out. You can also use Ammonia, but be in a well ventilated area and wear eye protection and avoid skin contact. And let it soak over night. The best way to this is to remove the bottom bracket, cover the end of the seat post so fluid won't come out, pour either coke or Ammonia into the bottom bracket so it pours down into the seat tube, meaning the frame will have to upside down and remain upside down over night.

If it's a steel post stuck in a steel frame then use PB Blaster the same way as above.

In the future use a small amount of grease on your fingertips and spread a very thin layer on BOTH the seat post and the inside of the seat tube down as far as the post would extend, then slide the post in and wipe off the excess grease. Any grease will work, I use black automotive bearing grease and never have an issue. Some people regrease once a year, I don't, grease lasts a long time, I may do it once every 5 years and still the grease is lubricating the seat post just fine, but to be safe I redo it. I had no problem pulling a AL seat post out of a steel bike that had been stored for 30 years and the grease was the original grease from the factory and it still allowed the seat post to come out easily.