Congratulation!
Do negotiation or not depends on many factors.
What is your targeted number?
What is the salary range for this position in this company?
You need to know these two factors.
The best way to negotiate is cal the decision maker, make a appointment, do
direct talk.
Some people successfully negotiated salary by email.
I have these sample emails on my website contributed by people who followed
my advise made the deal done.
Personally I prefer talking.
We also talked many times about how to negiated salary in our weekly FREE
meeting, download the meeting record if you need it is FREE too.
The bottom line is KEEP the OFFER!