How to practice openings

My ratings are  1500 in live std and around 1600 in CC. 

Till now my basic opening weapon is d4 with white and sicilian with black. I just want to know how you guys prepare openings. Specifically:

1. Do you memorize the opening lines or try to understand the basic principal behind them?
2. How do you practice them? Against computer or against friends or on chess.com live games?
3. I have Fritz 13 how do you suggest I can practice openings using its database?

Thanks 

1. The dumbest thing in the world is to memorize openings.  Take the King's Indian (since that's a 1.d4 opening).  Let's say you play the classical.  If you proceed to memorize 1.d4 Nf6 2.c4 g6 3.Nc3 Bg7 4.e4 d6 5.Nf3 O-O 6.Be2 e5 7.O-O Nc6 8.d5 Ne7 9.Ne1 Nd7 10.Be3 f5 11.f3 f4 12.Bf2 g5 and you select one of White's 13th move options.  If all you do is reel off the 12 moves, what do you do when Black plays 6...c5?  Or 7...Na6?  Or 7...exd4?  Or 9...Ne8?  Do you even have a remote clue as to what you are doing, or are you just mimicking moves like a monkey?

2. I might play a VERY LIMITED number of live games on here.  Otherwise, I mainly use them in rated over-the-board tournaments that are typically 1-day touranments.  Your minor, local events.  Ones that are say, 3 rounds, Game/90, or 3 Rounds, Game/75, or 4 Rounds, Game/60.  You WILL lose some games.  However, if you play it say, 20 times over the board, then when you go to a large tournament like the Chicago Open, World Open, Linares, etc, you are ready for it!

A recent book is far better than trying to teach it yourself via a database.  Check out www.everymanchess.com, www.qualitychess.co.uk, www.chess-stars.com, www.mongoosepress.com, etc.

Thanks Thiller,

Yeah I  don't memorize openings. I use d4 because I am playing it for some time and with all trials an errors I have some fair idea now abt where its variations lead.

I don't play OTB because of various issues. Generally I practice it against computers.

Thanks for the links I will try them

thnx for the information rishi.btw, what's OTB?

First to respond to your questions...

1. Both but mostly understanding.

2. Preferably tournament/longer online games

3. Don't practice against a computer it is not useful.

My suggestion. Get a basic understand by reading through the theory. Get a large database of games from strong players and observe mainly the common middlegame themes of your opening. Play this game as much as possible and learn from you mistakes. It is easiest to learn from an opening when you play it yourself!

Thx zkman. Will definitely try to implement ur suggestions.

@clc: OTB is  'Over the Board' mean chess not on internet but live.

1.Both

2.Chess.com live games

3.I suggest using Chess Position Trainer