first of here are some major rules when it comes to octopuses:
1.the tank must be completely escape proof. they can escape from tiny tiny holes (even 1/2" or smaller) here's a not bad example:
http://www.youtube.com/watch?v=9-azBDt0kik
2.know your octopus lifespan. the one thing about octopuses are they are very short life spanned. the average life span is a year, so dont plan on having it too long.
3.never underestimate the octopus. octopus are some of the smartest animals on earth. they learn very quick, and have been known to open valves, open containers, and much more. never ever underestimate the octopus.
4.to tell you the truth, from what i've seen and heard, octopuses aren't the funnest pet sometimes. they are known for hiding alot, so make sure you have plenty of liverock to make it comfortable.
5. you should make sure to have very very good water conditions before you get your octo, and make sure your water conditions aren't swinging through out the day.
go onto tonmo.com and go to the forums. its a forum just for cephalopods.
a good alternative in my opinion is sepia bandensis, aka the dwarf cuttlefish. they breed pretty easily, and are cute as hell, and a little easier than octopuses from what i've read.
hope it works out for you no matter what cephalopod you choose.