For Solaris create a boot script similar to this (e.g.: /etc/rc2.d/S99ndd)

#!/bin/sh
# increase max tcp window
# Rule-of-thumb: max_buf = 2 x cwnd_max (congestion window)
ndd -set /dev/tcp tcp_max_buf 4194304
ndd -set /dev/tcp tcp_cwnd_max 2097152

# increase DEFAULT tcp window size
ndd -set /dev/tcp tcp_xmit_hiwat 65536
ndd -set /dev/tcp tcp_recv_hiwat 65536

For more information, see this Solaris documentation.