Loose seatpost and other problems

ChrisNW Seatpost
I badly measured my seat tube, and got a size of 1" (25.4). Bought a seatpost, fitted it and found it difficult to get it tight enough.

After some checking and remeasurement, it appears my seat tube is oval by about 1mm, roughly 25.4 front to back and over 26mm side to side.

What should I do?
I've tried shimming the post out with a piece cut from and old paint tin, but it still slips.
I'm thinking to roughen it a bit but haven't tried that yet.

I don't want to splash out on another seatpost, any suggestions?

1. Coke can shim - but grease it up with some Copaslip or simlar to stop it seizing - this won't hold it, but it should keep it from wiggling too much and making the problem worse.

2. Jubilee Clip clamped tight around your seat post - no prizes for pretty but should work.