Choosing a new seat post question

My current seat post is the original that came with the bike and it's a brand that I find listed on ebay for around $8 a seat post. So, very cheap. This wouldn't be a problem except that my saddle will NOT remain in the same place that I put it and inevitably slowly works its way backwards. It's *extremely* annoying because of course I get the saddle just where I want it and a week later it's two inches out of place.

I have no idea how to chose a new seat post. I obviously know I need to make sure the measurement of the post fits my bike, and that I need to be able to attach my saddle by its rails, but other than that I am clueless noobie Can someone give me some info on what I need to do? Yes, I could go to my LBS but I do not particularly like the one nearby, so I don't want to go there.

I have a 1998 Raleigh SC-200, a comfort bike, and I am looking for a basic seat post that will hold my saddle securely. Not too cheap, not too expensive. I have a Selle Royal Lookin' saddle.

TIA!