Try to find out what the going rate is for your discipline and don't take anything less. This should be considerably more than what you would earn as a permie, if not then you're on a nice salary so don't bother with contracting
You can find the going rates in salary surveys done by agencies (I don't have a link sorry). Whatever you do don't take less than the going rate. You are not being employed, you are negotiating a business deal. Anything less than the going rate and you are being ripped off. Also don't be afraid to ask for more if you feel the job deserves it. Don't feel guilty, they will work you to the bone, treat you like a 2nd class citizen and get their monies worth.
As I've found out after a year of contracting there are a lot of negatives (both financially and in terms of rights) and that is why you will find the more experienced contractors pushing for higher rates.
Also it is important to factor in downtime, not just in between contracts but imminent unemployment will constantly loom over you. At any stage they can decide there's not enough work to warrant having you and you'll be dropped....be prepared for this. Set aside a months salary at least.