What is good seamanship?
The first reference to seamanship was in about 1760 and it was defined as the "knowledge and skill pertaining to the operation, navigation, management, safety, and maintenance of a ship".
Tom Cunliffe describes seamanship in his book as making sailing look easy - which leads nicely into my definition of "having the skills and knowledge and applying them to make an informed decision".
During your sail training courses like the RYA Day Skipper and RYA Coastal Skipper, and the Club Yachting courses I introduced Day Skipper Plus (bridging the gap between Day Skipper and Coastal Skipper; and Coastal Skipper Plus (which takes your skills level from the Coastal Skipper towards Yachtmaster Offshore) you will learn the skills to complete the courses and acquire knowledge.
A whichever level you are at, and whatever your goals, everyone likes to make what they do look easy. This is seamanship - the applications of your skills and knowledge by understanding your boat, how she manoeuvres in harbour and sails at sea.
I suggest reading the Tom Cunliffe book - its full of useful information to help you gain more understanding of seamanship and looking good.
There are also some great articles on seamanship in Cruising World which contain lots of useful information.
To have good seamanship you also need to take care of your environment - taking care what you dispose of overboard. Guidance on this can be found on the Green Blue website - an RYA initiative in conjunction with British marine industries.