What are good resources or techniques to improve one's offence or finishing game?
Get a book of checkmate problems, and go through the whole thing.  Then find another, and do the same thing.  When you are done, you will have no problem attacking.

Once your feel comfortable with simpler checkmate problems, get a book of general problems.  After all, in a real game, not every move is forced checkmate :) One of my favorites (which is extremely challenging) is Winning Chess Tactics Illustrated by I.A. Horowitz.
Another very good site for these is chesstempo.com, which will adjust the difficulty of the problems it shows you based on how well you do.

After you get a bit better (after, say, 5000 problems), if you are still interested in improving your attacking skills, I highly recommend Vladimir Vukovic's book Art of Attack in Chess.  Also, study the games of World Champions Tal and especially Alekhine, widely considered to be the greatest attackers of all time.