I really, really like the game, it's one of my top three of all time. It's also the only game I've bothered to get platinum on, I just adore the game that much. Also, I really can't wait for Dark Souls, the spiritual succcesor to the title, even if it's a long while from now. Because of that, I recently started yet another playthrough of the game. I just reached over 100 hours of playing the game in total, spread out over my old and the character I just started. It is a bit tough to start again, as I can feel I'm a bit rusty, but that just means I have a challenge on my hands all over again, which is damn awesome.
As for advice, as been said before, dodging is your friend. It's handy when facing mobs scattered throughout the level, but it's absolutely essential once you fight bosses, since blocking will most surely get you killed. Also, make sure that your equip weight is half of your equip limit. If you do that, you can dodge much, much faster, and as I said, it's all-important on bosses, where armor won't save you. Dodging will. Also, your stamina is something you should look out for, also very, very important.
Other than that, I have this extremely useful page: http://demonssouls.wikidot.com/ for you. It has information about pretty much everything you could want in the game and it's really well-written. It's actually made by the guys, who eventually ended up writing the manual for the European version of the game. If you have any question or anything at all, feel free to contact me, I know most, if not all ins and outs of that game.
Oh, and last but not least, we have a user group made up of people looking forward to Dark Souls. It's very nice group of people, most of them are also very good at Demon's Souls and all of them are a friendly lot. I hope that helps.