Seat Slippage Issues

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I had the same problem on my Bianchi mtb.
There should be a compression slot in the top of the seat tube where you insert the seat post (this slot allows the seat binder bolt/qr lever to squeeze the seat tube on the seat post). The simple and easy fix is to take a narrow file (like a jeweller's flat file), and very slightly widen the slot, little by little, until the seat binder bolt/lever holds the seat post firmly. If the frame is steel, you will then need to apply some anti-rust treatment (primer/anti-rust compound) to the slot.

edit: make sure there isn't excessive grease on the seat post/in the seat tube opening. You can also wrap some medium sandpaper (100 grit) around the seat post and gently roughen it around its diameter (i.e. not lengthwise) where it fits into the seat tube. This will give it a little extra grip.