I had a devil of a job on a replacement Genesis i0 frame. Never had the issue before in 15yrs of MTBing, tried various seat posts (they all vary slighly around the manufacturing tolerance), I tried heavy duty downhill seat clamps, I was going to try an oversize seatpost (27.3 instead of 27.2), in the end a shim made from a coke can wrapped around the seat post solved the problem and never had an issue since. Mind you, I probably won’t be able to get it out again 🙁
^ I am going to contradict the above advice and say:
Two clean smooth surfaces will be more likely to corrode. You don’t say what materials your frame and post are but tbh most combinations are likely to have some sort of galvanic corrosion.
My advice would be to clean the inside of the seat tube and the seat post with a suitable de-greaser – eg IPA, vodka, suitable bike wash etc.
Then either use a light smear of something like an inert anti-seize (not copper slip) or a very, very thin and light smear of assembly paste as mentioned above. Too much and you will get scratching the post but make sure you cover the whole inserted bit. I would start with a lentil sized blob and see how far you can spread it round.