Read 'Siblings without Rivalry' - lots of good tips.
On the whole a bit of healthy bickering is normal, but don't let it become a weapon to be used, to get more stuff. Having lots of things won't stop them arguing, or automatically make them happy children - in fact it might make them worse, if they suspect that their arguing is what makes you try and give them what they want, in order to keep the peace!
I try (my kids are 2 and 4yrs):
To encourage sharing by saying to them "It's good to share, isn't it?" whilst demonstrating it myself.
Try and get them to think up their own solutions to fighting over toys - "I can see you both really want to play with it at the same time, but that's not going to work. Can you come up with any good ideas, on how you can both play with it nicely?"
Sometimes I will simply take the offending toy away if it carries on, as it's quite a good reminder that there are boundaries, and I mean what I say.
Catching them doing nice things for each other (like when I see them giving each other a helping hand, or showing thought and consideration for one another - I try and praise their actions, quite specifically, so they know exactly what it was they did 'right'. "That was really nice of you, to think of your sister then!"
Try not to take sides or label - such as 'the naughty one' or 'the smart one'. Try and make sure they both know they're loved for their individual qualities.
Arrange bits of quality time to spend with each of them, without the other sibling if possible. Perhaps the arguing is because they're vying for attention, and focussing on things, is just a distraction from that.