How to improve ssh security?

I'm not really able to advise you on ssh security in general, although it's a good idea to make sure root can't log in remotely (check the sshd_config manpage -- it has plenty of good ideas!)

There are a few tools you could use to restrict attempts at brute force attacks -- I use fail2ban, which is available in Debian. I don't know if it's considered good or not, but it does the job for me.

--
A tidy house is the sign of a stolen computer.

You can use iptables firewall to restrict access to 22 port.
Also you can use parameter in sshd_config

MaxStartups
Specifies the maximum number of concurrent unauthenticated con-
nections to the sshd daemon. Additional connections will be
dropped until authentication succeeds or the LoginGraceTime
expires for a connection. The default is 10.

Alternatively, random early drop can be enabled by specifying the
three colon separated values ``start:rate:full'' (e.g.,
"10:30:60"). sshd will refuse connection attempts with a proba-
bility of ``rate/100'' (30%) if there are currently ``start''
(10) unauthenticated connections. The probability increases lin-
early and all connection attempts are refused if the number of
unauthenticated connections reaches ``full'' (60).

It's your solution :)
Good luck

Janet Kellman, software reviews