Know your machine, a good knowledge of the systems, fuel capacity and endurance. Be able to work out accurate payloads / endurance at different fuel loads off the top of your head.
Know the rules and operator SOP's well, You cant always pull out the books.
Listen to all the advice you can get, then decide whether you think its crap or not.
Probably the most dangerous phrase in G.A "she'll be right". she aint always right, often shes not even close.
My other favourite one is 'the last guy did it ok." (There is a joke in NZ that sums this one up quite well)
Expect to be put on the spot. Whether it be taking extra weight from a short strip or pushing into bad weather. Set your limits early and dont push beyond them. the pax may kick up a stink but safety has to be the over-riding factor at all times, its for their benefit too. if your getting worried then usually your pax started to worry 10 minutes ago.
Other than that I would suggest getting a taste for canned food and always carry a good book with you. At the end of the day GA is a fun sector of aviation, try and enjoy it.
Its only as dangerous as you choose to make it.