I see this is a super old thread

but that's okay.
My dd and I actually wear corsets on almost a daily basis. We wear Victorian style (not underbust) because we do living history reenacting (1850s-1860s Minnesota). We find them more comfortable than bras, and honestly women in the 1800s did everything we do now while wearing them.
A few things -- the plastic boning won't last nearly as long as steel boning, and spiral steel is very flexible. Plastic boning won't give as good support, and it will mold to your body over time because of your body heat.
For the most comfortable corset, make it yourself, don't buy off the rack. They are somewhat time consuming to make -- it took us about 10 hours total time to make each of ours -- but once it's done it's done. You should wear something underneath it (period accurate was a chemise) to protect it from your body oils. You will very, very rarely (if ever!) have to wash it if you do this.
There is no way I can wear an underbust corset -- I am a nursing mama with two kids, and an underbust corset just doesn't support me.
I buy all my supplies from
http://corsetmaking.com/ It cost me about $100 to make 2 corsets, and future ones will be less because I already have the pattern and some knowledge so won't buy the wrong size of bones.
No matter which pattern you go with, do a thorough websearch for advice about that particular pattern. They can be sewn by hand -- my dd at 16 years old sewed hers almost completely by hand.
The hanging-onto-the-bedpost-to-get-it-tighter is all lies. A corset should offer firm support but not be uncomfortable. If it is, it's either too tight or doesn't fit right. It shouldn't dig in to your hips/lap or shove your bust to your chin when you sit. It shouldn't slide down throughout the day, and you should be able to breath freely.
My daughter right now is wearing hers over a chemise while we are washing her dress. She simply finds hers so much more comfortable than a bra that she wears it as often as she can.