nmap options/ideas to avoid Firewall

Packet fragmentation 

-f 

Modify default MTU: 

--mtu 24 

Random number of decoys 

-D RND:10 

Change Source port:

--source-port 80 

Append Random Data to Packet: 

--data-length 25 

MAC Address Spoofing 

--spoof-mac Dell/Apple/3Com